ELEMENTS 

OF 

RADIO  TELEPHONY 


— "—- 


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ELEMENTS 

OF 

RADIO  TELEPHONY 


BY 


WILLIAM  C.  BALLARD,  JR.,  M.E. 

ASSISTANT  PROFESSOR  OF  ELECTRICAL  ENGINEERING 
CORNELL  UNIVERSITY 


FIRST  EDITION 


McGRAW-HILL  BOOK  COMPANY,  INC. 

NEW  YORK:   370  SEVENTH  AVENUE 

LONDON:  6  &  8  BOUVERIE  ST.,  E.  C.  4 

1922 


COPYRIGHT,  1922,  BY  THE 
MCGRAW-HILL  BOOK  COMPANY,  INC 


PREFACE 


The  phenomenal  popularity  into  which  radio  telephonic 
broadcasting  has  sprung  has  been  the  means  of  interesting 
thousands  in  radio  transmission.  To  the  non-technical 
reader  the  transmission  of  speech  and  music  with  no  visible 
means  of  intercommunication  is  somewhat  of  a  mystery. 
While  it  is  mysterious  it  is  no  more  mysterious  than  the 
production  and  recognition  of  light,  for  radio  and  light  waves 
are  of  exactly  the  same  nature.  The  relation  of  radio  and 
light  waves  is  the  same  as  that  existing  between  red  and  blue 
light,  it  is  merely  a  matter  of  frequency.  Of  course  the 
"  colors  "  of  the  radio  spectrum  are  invisible  as  far  as  the 
human  eye  is  concerned  and  the  radio  receiver  is  nothing 
more  than  an  artificial  eye  sensitive  to  these  extremely  low- 
frequency  "  colors." 

This  little  volume  has  a  three-fold  purpose :  first,  to  pre- 
sent in  simplified  form  a  brief  discussion  of  what  happens 
when  messages  are  sent  and  received  by  radio;  secondly  a 
brief,  simplified  description  of  the  apparatus  required  to 
produce  these  effects  and  how  it  operates;  and  lastly,  prac- 
tical unbiased  information  for  the  experimenter  who  desires 
certain  results  but  who  does  not  know  what  apparatus  is 
necessary. 

The  use  of  mathematics  has  been  almost  entirely  avoided 
and  the  treatment  in  most  cases  is  qualitative  rather  than 
quantitative.  For  those  readers  who  are  interested  in  the 
calculation  of  the  numerical  constants  of  the  circuits  shown 
in  the  book,  Circular  No.  74  of  the  Bureau  of  Standards 
vii 

2OG8192 


viii  PREFACE 

entitled  "  Radio  Instruments  and  Measurements  "  will  be 
found  to  contain  valuable  material.  This  circular  may  be 
obtained  from  the  Superintendent  of  Documents,  Govern- 
ment Printing  Office,  Washington,  D.  C. 

The  usual  conventions  have  been  used  in  the  circuit 
diagrams.  For  the  sake  of  simplicity  filament-regulating 
resistances  have  been  omitted  from  all  tube  diagrams.  The 
diagrams  illustrating  the  currents  and  voltages  in  detector 
circuits  are  not  intended  to  represent  oscillograph  records 
but  have  been  conventionally  represented. 

The  author  is  indebted  to  his  colleague  B.  K.  Northrop 
for  many  valuable  suggestions  in  the  preparation  of  the 
manuscript  and  to  the  American  Radio  and  Research 
Corporation,  the  General  Electric  Company,  the  Westing- 
house  Electric  and  Manufacturing,  and  the  Mullard  Radio 
Valve  Co.,  London,  England,  for  photographs  of  radio 
apparatus. 

W.  C.  B. 

CORNELL  UNIV.,  ITHACA,  N.  Y. 
May,  1922. 


CONTENTS 

PAGE 

PREFACE     v 

CHAPTER  I 

WIRE  AND  RADIO  TELEPHONE  SYSTEMS 1 

The  Telephone  Receiver  and  Transmitter — Comparison 
between  the  Two  Systems. 

CHAPTER  II 

HIGH-FREQUENCY  CURRENTS  AND  THEIR  PRODUCTION    ...       6 
Alternating  Currents — Sound,  Heat  and  Light  and  Their  Rela- 
tion to  Radio  Waves — Damped  and  Sustained  Waves — Spark 
Transmitters — Arc  Transmitters — Alternator  Transmitters — 
Tube  Transmitters. 

CHAPTER  III 

VACUUM  TUBES 14 

The  Physics  of  Electron  Emission  from  the  Filament — Flem- 
ing Valve — Space  Charge — Grid  Electrode  and  Its  Effect — 
Tungsten  and  Coated  Filaments. 

CHAPTER  IV 

VACUUM  TUBE  OSCILLATORS 24 

Mechanical  Examples  of  Oscillation — Self  -excited  Tube  Cir- 
cuits— Various  Systems  of  Feedback  Coupling — Wave  Length 
of  Resultant  Osculations — Plate  Voltage  Supply  Systems — 
Direct  current  Generators — Rectifier  Systems — Hot  Cathode 
Rectifiers— Cold  Cathode  Rectifiers— Electrolytic  Rectifiers- 
Filter  Condensers — Electrolytic  Condensers — Master  Oscillator 
and  Power  Amplifier. 


x  CONTENTS 

CHAPTER  V 

PAGE 

MODULATOR  SYSTEMS 47 

Function  of  Modulator— Modulation  by  Antenna  Control- 
Microphone  in  Antenna  Circuit — Magnetic  Modulator — 
Modulation  by  Variation  of  Tube  Output— Plate  Circuit 
Modulation — Grid  Circuit  Modulation. 

CHAPTER  VI 

RECEIVING  EQUIPMENT 55 

Sympathetic  Vibration — Radio  Tuning — Electrical  Equiva- 
lent of  the  Antenna — Crystal  Detector  Receivers — Loose 
Coupler  Circuits — Vacuum  Tube  Detector — Action  with 
and  without  Grid  Condenser — "Soft"  Tubes — Regenerative 
Receivers — Single  Circuit  Receiver — Coupled  Circuit  Re- 
ceiver— Methods  of  Adjustment — Audio  Frequency  Amplifiers 
— Radio  Frequency  Amplifiers — Loud  Speakers — "Push-Pull" 
Amplifier  Circuit. 

CHAPTER  VII 

TRANSMISSION ' 98 

The  Antenna — Production  of  Ether  Waves — Relation  between 
Frequency  and  Wave  Length — Elevated  Antennae — Antenna 
Construction — Masts — Wire — Ground  Systems — Loop  or  Coil 
Antennae — Transmission  Range — "Fading" — Atmospherics  or 
"Static." 

CHAPTER  VIII 

MISCELLANEOUS       .       .       .       .       „       .       .       .       .       .       .116 

Storage  Batteries — Lead  Acid  Batteries — Nickel  Iron  Batteries 
— Plate  Circuit  Batteries — Storage  Battery  Charging  Appa- 
ratus— Wavemeters — Measurement  of  Transmitter  Wave 
length — Receiver  Circuit  Calibration — Selection  of  Apparatus 
for  Broadcasting  Reception — Range  of  Various  Sets — Indoor 
Antennae — Equipment  Using  Loop  Antennae — Loud  Speakers. 


ELEMENTS  OF  EADIO  TELEPHONY 


CHAPTER  I 
WIRE  AND  RADIO  TELEPHONE  SYSTEMS 

Wire  and  Wireless  Telephone  Systems. — The  produc- 
tion of  sound  depends  upon  setting  the  air  into  suitable 
vibration.  When  we  speak  the  vocal  chords  or  membranes 
located  down  in  the  throat  vibrate  and  set  the  column  of  air 
in  the  throat  into  vibration  and  these  vibrations  are  in 
turn  transmitted  into  the  surrounding  air.  The  air  waves 
thus  formed  beat  upon  the  membrane  of  the  ear  and  cause 
it  to  vibrate  in  the  same  manner  as  the  vocal  chords  did  to 
produce  them.  The  vibration  of  the  membrane  of  the  ear, 
or  the  ear  drum  as  we  more  commonly  call  it,  affects  the 
nerves  of  the  ear  and  the  message  is  in  turn  transmitted  to 
the  brain. 

Transmission  over  wire  or  wireless  systems  is  quite  sim- 
ilar to  the  above  procedure  but  instead  of  using  air  waves, 
waves  of  electric  current  are  employed.  Fig.  1  shows  a 
typical  telephone  circuit  which  for  the  sake  of  simplicity  is 
arranged  to  transmit  only  in  one  direction.  M  is  a  tele- 
phone transmitter  or  microphone  as  it  is  sometimes  called. 
B  is  a  battery  which  supplies  the  force  necessary  to  send  the 
current  around  the  electric  circuit  comprising  the  line  wires 
LI  and  Lz;  R  is  the  telephone  receiver.  The  essential  parts 


2  ELEMENTS  OF  RADIO  TELEPHONY 

of  a  telephone  transmitter  are  a  thin  sheet  of  material, 
usually  metal,  called  the  diaphragm,  which  is  arranged  so 
as  to  be  set  into  motion  when  sound  waves  impinge  upon  it; 
and  some  type  of  variable  resistance  contact  arranged  to  be 
operated  by  the  motion  of  the  diaphragm.  The  variable 
resistance  contact  usually  consists  of  a  small  chamber  in 


L, 


Wire  Telephone 

L, 


/L 


Battery       Modulator 


Line  Wires          Receiver 
Antennae  ReceivingSet 


n 

1 

T 

-*—  «  1^ 

Radio  Telephone 

"O\ 

_o/ 


Ground. 
Fia.  1.— Graphical  comparison  of  wire  and  radio  telephone  systems. 

which  two  carbon  discs  are  situated  and  insulated  from  one 
another.  One  disc  is  stationary  and  the  other  is  attached 
to  the  diaphragm  and  free  to  move  with  it.  The  space 
between  the  two  discs  is  loosely  packed  with  carbon  grains 
about  the  size  of  a  pin  head.  The  carbon  granules  have  the 
property  of  varying  the  electrical  resistance  between  the  two 
carbon  discs,  which  resistance  depends  upon  the  degree  of 


WIRE  AND  RADIO  TELEPHONE  SYSTEMS  3 

pressure  applied  to  the  granular  carbon.     When  the  carbon 
granules  are  tightly  compressed  their  resistance  is  low  and 


diaphragm 

carbon  granules 


diaphragm. 


cose 


permanent  mognek 
electro  magnet's. 


FIG.  2. — Telephone  transmitter  or  microphone  and  telephone  receiver 
removed  from  their  cases  to  show  internal  construction. 


this  allows  the  battery  to  send  a  comparatively  large  current 
through  the  circuit.    When  pressure  is  removed  the  resistance 


4  ELEMENTS  OF  RADIO  TELEPHONY 

increases  and  the  current  is  correspondingly  reduced.  Thus 
the  sound  waves  impressed  on  the  transmitter  diaphragm 
are  reproduced  in  the  electrical  circuit  as  changes  in  the 
strength  of  the  electric  current. 

The  receiver  consists  of  a  thin  sheet  of  soft  iron  so  arranged 
as  to  be  attracted  by  an  electromagnet.  Sometimes  this 
magnet  is  a  combination  of  permanent  and  electromagnet 
and  sometimes  the  permanent  magnet  is  omitted.  The  coil 
of  wire  wound  around  the  iron  core  constituting  the  ete 
tromagnet  has  the  property  of  attracting  the  diaphragm 
with  varying  degrees  of  force  depending  upon  the  current 
through  the  coil.  Since  the  diaphragm  is  flexible  it  will 
move  in  response  to  these  changes  in  pull  and  thus  send  out 
an  air  wave  which  resembles  to  a  greater  or  less  degree  the 
current  through  the  coil.  The  essential  features  in  wire 
telephony  are  a  source  of  power,  the  battery;  a  modulator 
of  the  power  source  so  that  the  output  varies  according  to 
the  sound  to  be  transmitted,  in  this  case  the  transmitter  or 
microphone;  means  for  transmitting  the  modulated  power 
output  from  the  transmitting  to  the  receiving  station, 
represented  by  the  line  wires;  and  finally  a  device  to  change 
the  electrical  impulses  back  to  sound  waves,  which  is  t 
function  of  the  receiver. 

The  operation  of  radio  or  wireless  telephone  systems 
requires  corresponding  units  similar  to  those  already  enumer- 
ated and  which  have  exactly  the  same  functions.  The 
principal  point  of  difference  is  that  in  order  to  transmit 
power  through  the  ether,  which  we  have  to  employ  in  place 
of  the  line  wires,  it  is  necessary  to  use  high-frequency  energy, 
and  hence  the  power  source  will  have  to  be  one  capable  o 
producing  high-frequency  currents  and  the  receiving  system 
sensitive  to  high-frequency  currents  instead  of  currents 
such  as  are  produced  in  wire  telephony.  Thus  for  radio 
telephony  the  battery  will  be  replaced  by  a  high-frequency 


WIRE  AND  RADIO  TELEPHONE  SYSTEMS  5 

current  generator,  the  microphone  by  a  modulation  system, 
the  line  wires  by  the  two  antennae  and  the  intervening  ether, 
and  the  receiver  by  special  receiving  apparatus  sensitive  to 
high-frequency  currents. 

Fig.  1  shows  these  relations  in  diagrammatic  form. 


CHAPTER  II 

HIGH-FREQUENCY    CURRENTS    AND    THEIR    PRO- 
DUCTION 

The  current  produced  in  the  wire  telephone  circuit  shown 
in  Fig.  1  is  what  is  known  as  a  direct  current  since  it  flows  al- 
ways in  the  same  direction.  Currents  of  this  general  type  are 
used  extensively  in  electrical  work,  street  cars  are  almost 
exclusively  operated  on  direct  current.  Electric  lights  are 
usually  supplied  with  alternating  current  for  certain  tech- 
nical reasons.  Alternating  currents  do  not  flow-in  any  given 
direction  but  reverse  their  direction  of  flow  many  times  per 
second.  A  change  from  zero  through  a  series  of  positive  or 
negative  values  back  to  zero  again  is  spoken  of  as  an  alter- 
nation and  two  alternations  compose  a  cycle.  The  number 
of  cycles  passed  through  per  second  is  the  frequency.  These 
different  values  are  illustrated  in  Fig.  3,  which  illustrates  the 
variation  of  a  60-cycle  current  with  tune.  It  makes  no  dif- 
ference where  we  start  or  end  a  cycle  or  alternation;  these 
values  are  essentially  units  of  measurement  and  applicable 
equally  well  at  any  starting  point. 

The  frequency  most  commonly  used  on  alternating-cur- 
rent power  and  lighting  circuits  is  60,  and  if  the  filaments 
of  the  incandescent  lamps  could  cool  off  rapidly  enough  they 
would  actually  go  out  120  times  a  second  or  every  tune  the 
current  passed  through  zero.  However  the  filament  usually 
stores  up  enough  heat  to  supply  it  over  the  period  of  zero 
current  and  we  do  not  notice  any  appreciable  flicker.  In 


HIGH-FREQUENCY  CURRENTS  7 

spite  of  the  heat  storage  of  the  filament,  flickering  is  fre- 
quently noticeable  on  15-  and  25-cycle  circuits. 

The  frequencies  used  in  radio  work  are  very  much  higher 
than  those  employed  in  power  work.  Radio  frequencies 
range  from  about  15,000  cycles  up  to  several  millions  of 
cycles  per  second. 

Sound  waves  are  alternating  air  currents  or  disturbances 
in  the  air  while  radio  waves  are  alternating  disturbances  in 
the  "  ether."  Sound  waves  are  mechanical  disturbances 


seconds 


FIG.  3. — Alternating-current  wave  of  60  cycles  frequency. 


while  radio  waves  are  electromagnetic  disturbances.  Light 
and  heat  are  electromagnetic  "  ether  "  disturbances  similar 
to  radio  waves  but  of  very  much  higher  frequencies.  Fig.  4 
has  been  drawn  with  the  idea  of  comparing  the  various 
sound  and  electromagnetic  waves  with  which  we  have  to 
deal.  A  piano  keyboard  comprises  about  seven  octaves. 
The  frequencies  of  sound  waves  corresponding  to  notes  one 
octave  apart  bear  the  relation  to  two  to  one.  If  we  double 
the  frequency  of  any  given  note  we  always  get  the  octave 
abov^e  it.  The  drawing  shows  an  enlarged  keyboard  with 
the  octaves  indicated,  it  should  be  very  definitely  remem- 


8  ELEMENTS  OF  RADIO  TELEPHONY 

bered  that  although  marked  on  the  same  scale,  that  there  is  a 
fundamental  difference  between  sound  and  radio,  light,  and 
heat  waves  other  than  that  of  difference  in  frequency.  As 
we  increase  the  frequency  from  the  audible  range  of  the  piano 
we  reach  the  limit  to  which  the  ear  responds  at  about  10,000 
cycles  per  second  and  slightly  beyond  this  value  we  reach 
the  longest  radio  waves  in  use  to-day.  The  upper  limit  of 
radio  frequencies  in  ordinary  use  is  about  four  million  cycles 
and  from  this  point  to  the  longer  heat  waves  is  a  region 
utilized  only  in  laboratory  experiments.  As  the  heat 
waves  become  of  higher  frequency  they  pass  into  visible  light 

/  *  M  I  I  I  I  I  I  M  M  I  I  I  l'  I  I  f  /- c^  *,*»*  -/I        f       f   / 


HCAT      ii6HT    'x't&ra 


FIG.  4. — Chart  showing  the  relative  frequencies  of  sound,  radio,  and 
light  waves. 


starting  at  the  red  end  of  the  "  rainbow  "  or  spectrum  and 
finally  passing  into  invisibility  through  the  blue  end  of  the 
spectrum.  Invisible  waves  of  higher  frequency  than  the 
blue,  while  having  no  visible  effect  on  the  eye,  will  affect  a 
photographic  plate  and  produce  other  manifestations.  As 
we  go  still  higher  we  come  upon  the  very  short  waves  of  high 
penetrative  power  known  as  X-rays.  These  rays  are  invis- 
ible to  the  eye  but  are  capable  of  affecting  photographic 
plates  and  also  produce  luminous  effects  when  impinging 
upon  certain  chemicals. 

Radio  and  light  waves  are  identical  except  for  the  differ- 
ence in  frequency;  it  so  happens  that  the  human  eye  is  sensi- 


HIGH-FREQUENCY  CURRENTS  9 

five  to  a  rather  restricted  band  of  frequencies,  otherwise  we 
would  be  able  to  see  radio  signals  as  flashes  of  light.  Radio 
telegraph  signals  would  be  recognized  as  alternate  flashes  of 
light  followed  by  periods  of  total  darkness  and  radio  tele- 
phone signals  would  correspond  to  the  light  given  off  by  a 
lamp  which  flickered  but  never  went  out  entirely.  As  will 
be  noticed  there  are  several  large  bands  almost  entirely 
unexplored  and  it  is  a  matter  of  conjecture  just  what  would 


V 

Damped  Wave 


FIG.  5. — Sustained  or  continuous  and  damped  waves. 

be  found  if  radio  receiving  sets  could  be  tuned  to  these  par- 
ticular ranges. 

Damped  and  Sustained  Waves.— There  are  two  general 
types  of  alternating  currents  useful  in  radio  transmission, 
(a)  those  producing  sustained  waves  and  (6)  those  producing 
damped  waves. 

In  the  sustained  type  the  currents  vary  between  constant 
positive  and  negative  maximum  values,  each  wave  being 
just  as  high  as  its  neighbor;  while  in  the  damped  type  the 
waves  become  smaller  and  smaller  as  time  goes  on.  For 


10  ELEMENTS  OF  RADIO  TELEPHONY 

instance,  an  organ  pipe  produces  a  sustained  sound  wave 
while  a  piano  string  produces  a  damped  sound  wave  which 
constantly  decreases  in  amplitude.  The  difference  between 
these  two  types  is  illustrated  in  Fig.  5.  Damped  waves  in 
general  are  not  of  particular  interest  in  radio  telephony, 
their  principal  application  being  to  radio  telegraphy. 

There  are  several  different  schemes  suitable  for  the  pro- 
duction of  high-frequency  currents  applicable  to  radio  com- 
munication, some  are  suitable  for  both  radio  telegraphy  and 
telephony  while  others  are  adaptable  to  radio  telegraphy 
alone. 

The  table  below  indicates  some  of  the  more  common 
systems  and  their  principal  applications. 

Spark  (audible  frequency)     Telegraphy. 
Spark  (inaudible  frequency)  Telegraphy  and  telephony. 
Arc  Telegraphy  and  telephony.  (?) 

Alternator  Telegraphy  and  telephony. 

Vacuum  tube  Telegraphy  and  telephony. 

Spark  Systems. — From  a  numerical  standpoint  at  least, 
the  spark  is  probably  the  most  important  system  in  use  in 
medium  and  short  distance  radio  telegraphy.  A  majority 
of  radio-equipped  vessels  operate  on  the  spark  system, 
although  it  may  be  expected  that  it  will  be  superseded  in  the 
near  future  on  account  of  certain  inherent  difficulties  in  the 
system. 

Since  this  system  is  only  difficultly  applicable  to  radio 
telephony  it  will  be  but  briefly  discussed  here.  It  is  possible 
to  set  up  tuned  electrical  circuits  in  the  same  way  that 
we  produce  tuned  conditions  in  musical  instruments.  If  we 
tiishtrii  the  strings  on  a  piano  up  to  the  proper  point  they 
will  sound  a  certain  tone  when  set  into  vibration,  each  tone 
corresponding  to  a  definite  number  of  cycles  per  second. 


HIGH-FREQUENCY  CURRENTS  11 

The  string  will  continue  to  vibrate  a  few  seconds  after  being 
struck  and  will  gradually  die  down  to  rest.  The  string 
vibrates  because  it  has  both  elasticity,  which  allows  it  to  be 
stretched  taut  on  the  sounding  board,  and  inertia  due  to 
the  mass  of  the  metal  composing  the  wire.  The  electrical 
circuit  on  the  other  hand  will  have  capacitance  and  induc- 
tance corresponding  to  elasticity  and  mass  in  the  piano 
string.  In  order  to  set  it  into  sudden  vibration  we  insert  a 
spark  gap  in  the  circuit  and  charge  the  condenser  up  to  the 
point  where  the  gap  breaks  down  and  allows  a  spark  to  pass. 
The  spark  which  passes  is  not  a  direct-current  discharge  as 
might  be  expected,  but  corresponds  to  a  high-frequency 
alternating  current  of  a  frequency  determined  from  the 
capacitance  and  inductance  of  the  circuit.  This  is  anal- 
ogous to  the  case  of  the  piano  string  where  the  frequency  of 
the  sound  vibration  is  determined  by  the  tension  and  weight 
of  the  string. 

The  usual  arrangement  is  to  have  the  high  voltage  sup- 
plied from  a  transformer,  in  which  case  we  ordinarily  get  one 
spark  per  alternation,  or  two  sparks  per  cycle.  If  the  gen- 
erator frequency  is  500,  as  is  the  case  in  most  marine  installa- 
tions, there  will  be  a  thousand  spark  discharges  per  second, 
one  for  each  positive  and  negative  peak.  At  each  discharge 
through  the  air  gap,  a  high-frequency  current  of  several 
cycles  gradually  diminishing  down  to  zero,  is  set  up,  and  by 
proper  connection  to  the  antenna  circuit  a  corresponding 
ether  wave  is  sent  out.  By  proper  arrangement  of  receiv- 
ing apparatus  at  the  receiving  antenna  a  note  of  1000  cycles 
will  be  heard  in  the  telephones. 

The  transmitter  at  Arlington  which  sends  daily  time  sig- 
nals is  operated  on  this  system. 

The  only  way  in  which  this  type  of  transmitter  can  be 
used  for  radio  telephony  is  to  increase  the  spark  rate  until 
the  spark  note  comes  above  the  limit  of  audibility  and 


12  ELEMENTS  OF  RADIO  TELEPHONY 

under  these  conditions  the  output  of  the  system  may  be 
modulated  for  radio  telephony  as  described  in  a  later  chap- 
ter. The  system  is  not  of  great  practical  importance  as 
other  systems  are  superior  to  it  in  very  many  respects. 

Arc  Systems. — Arc  transmitters  operating  on  the  Poulsen  — 
system  are  in  extbnsive  use  for  long  distance  telegraph  trans- 
mission and  are  gradually  being  introduced  in  smaller  sizes 
for  marine  work  in  place  of  spark  transmitters.  The  elec- 
trical principles  involved  are  not  greatly  different  from  those 
involved  in  spark  transmitter  operation.  In  normal  arc 
operation  one  spark  passes  for  each  cycle  of  high-frequency 
current,  while  in  spark  transmitters  there  are  several  cycles 
for  each  spark.  The  arc  is  supplied  with  direct  current 
instead  of  alternating  as  in  the  previous  case,  and  produces 
sustained  or  undamped  waves  in  contrast  with  the  damped 
wave  produced  by  the  spark  transmitter. 

The  arc  is  not  well  suited  for  radio  telephony  on  account  of 
hissing  produced  by  irregularities  of  the  transmitted  wave 
due  to  the  instability  of  the  arc. 

Alternator  Systems. — Alternator  systems  employ  appa- 
ratus of  the  same  general  nature  as  that  used  to  supply  power 
for  power  and  lighting  applications  except  that  instead  of 
generating  frequencies  around  60,  such  as  are  used  for  power 
applications,  they  must  be  capable  of  producing  frequencies 
above  10,000  cycles  per  second.  Several  different  alternator 
systems  have  been  developed,  among  which  should  be  men- 
tioned the  Alexanderson  inductor  type  in  which  the  high 
frequency  is  produced  directly  in  the  alternator,  the  Gold- 
schmidt  system  in  which  a  relatively  low  frequency  is  ini- 
tially generated  in  the  alternator  and  reflected  back  and  *" 
forth  several  times  between  the  two  windings  on  the  machine 
with  a  corresponding  increase  of  frequency  at  each  reflec- 
tion, and  the  Joly-Arco  system  in  which  the  frequency  is  — 
stepped  up  in  outside  stationary  apparatus. 


HIGH-FREQUENCY  CURRENTS  13 

Any  of  the  above  systems  produces  an  output  admirably 
suited  for  radio  telephony  and  is  subject  only  to  the  diffi- 
culties involved  in  controlling  the  output  in  accordance  with 
the  sound  wave  which  it  is  desired  to  modulate.  A  magnetic 
modulator  has  been  developed  in  connection  with  the  Alex- 
anderson  system  and  very  successful  tests  have  been  carried 
out  by  its  use. 

Tube  Systems. — The  three-element  electron  tube  variously 
known  as  the  audion,  pliotron,  triode,  etc.,  seems  to  offer 
the  ideal  solution  of  the  radio  telephone  transmission  prob- 
lem, and  the  results  already  accomplished  indicate  an 
immense  field  for  its  further  application.  In  fact  the  tube 
may  eventually  displace  all  other  systems  and  become  the 
one  universal  type  of  radio  apparatus.  On  account  of  its 
wide  application  both  in  receiving  and  transmitting  circuits, 
its  construction  and  operation  will  be  treated  in  a  separate 
chapter. 


CHAPTER  III 
VACUUM  TUBES 

It  has  been  known  in  scientific  circles  for  several  hundred 
years  that  heated  bodies  exhibit  peculiar  electrical  effects. 
For  example,  early  investigators  found  that  by  bringing  a 
charged  electroscope  near  a  red-hot  iron  ball  the  electroscope 
would  lose  its  charge.  As  a  part  of  his  researches  in  connec- 
tion with  the  development  of  the  incandescent  lamp,  Edison 
noted  that  the  filament  had  the  power  of  giving  off  electricity 
when  heated. 

Modern  discoveries  in  physics  indicate  that  all  material 
bodies  are  composed  of  minute  divisions  of  matter  termed 
molecules  and  that  these  molecules  are  in  constant  motion 
inside  the  substance.  The  rapidity  with  which  these  par- 
ticles move  and  the  temperature  of  the  body  are  thought  to 
be  closely  related,  the  higher  the  temperature  the  greater 
the  molecular  velocity.  On  the  other  hand  as  the  tempera- 
ture is  lowered  the  molecular  activity  becomes  less  and  less 
until  the  molecules  come  to  a  standstill  at  the  absolute  zero 
of  temperature  which  corresponds  to  273  degrees  below  zero 
in  the  Centigrade  scale. 

If  the  molecular  structure  of  a  material  is  such  that  elec- 
tricity can  pass  through  it,  we  call  it  an  electrical  conductor. 
Scientists  think  of  electricity  as  being  composed  of  large 
numbers  of  infinite  divisions  which  have  been  termed 
"  electrons."  An  electron  is  quite  a  small  value  when  com- 
pared with  the  amounts  of  electricity  we  deal  with  in  every- 
14 


n 


Vacuum  tubes  of  English  manufacture.  A  and  B  are  ''soft"  detector  tubes, 
C  and  D  are  hard  amplifier  tubes.  E  is  a  500-watt  transmitting  tube 
and  F  is  a  500-watt  rectifying  tube.  The  special  construction  of  E  and 
F  allows  the  filament  to  be  replaced  without  destroying  the  whole  tube. 


VACUUM  TUBES  15 

day  life;  for  example,  in  order  to  light  an  average-sized  incan- 
descent lamp  of  fifty  watts  capacity  it  requires  a  stream 
of  about  3,000,000,000,000,000,000  electrons  per  second. 

When  the  molecular  structure  of  any  material  prevents 
the  passage  of  electrons  more  or  less  completely  we  rate  this 
material  as  an  electrical  insulator. 

When  water  is  exposed  to  the  air  a  certain  amount  of  it 
vaporizes,  and  it  evaporates  as  we  say.  The  rapidity  with 
which  it  evaporates  is  dependent  upon  several  conditions, 
one  of  which  is  the  temperature  of  the  water.  This  is 
explained  according  to  the  "  molecular  motion  "  theory  of 
heat  by  saying  that  some  of  the  water  molecules  have 
attained  sufficient  speed  due  to  the  temperature  to  break 
through  the  surface  and  be  shot  off  into  the  air.  Water 
does  not  have  to  be  in  liquid  form  to  evaporate;  it  is  a  well- 
known  fact  that  ice  will  evaporate  at  temperatures  consid- 
erably below  the  freezing  point.  This  fact  can  be  checked 
by  hanging  a  wet  cloth  out  on  a  very  cold  day;  if  conditions 
are  right  it  will  dry  almost  as  rapidly  as  in  the  summer. 

Other  solid  substances  act  in  the  same  way  as  ice  but  usu- 
ally require  rather  high  temperatures  before  much  evapora- 
tion takes  place.  One  example  is  the  gradual  blackening 
of  incandescent  lamps  which  have  been  used  for  some  time, 
the  high  temperature  at  which  the  filament  is  operated 
causes  a  certain  amount  of  evaporation  which  condenses 
upon  the  cool  walls  of  the  glass  bulb. 

Tungsten  is  particularly  suitable  for  lamp  filaments 
because  it  can  be  heated  very  hot  and  give  off  large  quan- 
tities of  light  without  appreciable  evaporation. 

If  high  temperature  produces  extreme  velocities  of  the 
molecules  composing  the  material  it  is  reasonable  to  suppose 
that  it  may  also  be  true  with  regard  to  whatever  electrons 
happen  to  be  in  the  material.  As  the  temperature  is  in- 
creased more  and  more  some  electrons  will  attain  a  sufficient 


16  ELEMENTS  OF  RADIO  TELEPHONY 

velocity  to  break  through  the  surface  of  the  material  and  be 
shot  off  into  space.  This  idea  of  "  evaporation  "  of  elec- 
tricity from  heated  bodies  is  further  substantiated  by  the 
fact  that  the  numerical  relation  between  temperature  and 
amount  of  evaporation  are  very  much  the  same  in  the  case 
of  the  actual  evaporation  of  the  material  and  "  evaporation  " 
of  the  electrons  or  thermionic  emission,  as  it  is  usually  called. 

It  has  been  found  that  the  electricity  given  off  from  heated 
filaments,  such  as  are  used  in  electron  tubes,  is  negative  as 
compared  with  the  commonly  accepted  idea  of  positive 
and  negative  electricity.  Years  ago  the  assumption  was 
made  that  an  electric  current  consisted  of  a  motion  of 
electricity  from  positive  to  negative.  This  was  an  arbi- 
trary assumption  made  on  account  of  a  lack  of  information 
one  way  or  the  other,  and  it  seems  now  that  it  was  incor- 
rect for  we  believe  that  the  electron  passes  from  negative  to 
positive  and  has  the  same  effect  as  a  passage  of  so-called 
"  positive "  electricity  in  the  opposite  direction.  Thus, 
according  to  the  original  assumption  an  electron  is  negative 
electricity  and  passes  from  negative  to  positive. 

Fleming  Valve. — The  earliest  application  of  the  thermionic 
emission  principle  to  radio  telegraphy  was  in  the  Fleming 
valve  used  as  a  rectifier  in  radio  receiving  circuits.  The 
apparatus  consisted  of  an  incandescent  lamp  filament 
mounted  together  with  another  electrode  in  the  form  of  a 
plate  inside  a  glass  bulb  and  exhausted  in  the  usual  manner. 
When  the  filament  is  brought  up  to  incandescence  large 
numbers  of  electrons  are  shot  off.  As  each  electron  carries  a 
negative  charge,  the  removal  of  electrons  or  negative  elec- 
tricity imparts  a  corresponding  positive  charge  to  the  fila- 
ment, the  removal  of  negative  electricity  having  the  same 
effect  as  the  addition  of  positive  electricity.  Unlike  polar- 
ities attract  and  like  polarities  repel,  so  the  positive  filament 
will  attract  back  all  the  electrons  unless  some  other  elec- 


VACUUM  TUBES  17 

trical  force  is  acting  upon  them.  However,  if  the  plate  is 
charged  positively  by  connecting  it  to  a  battery,  a  certain 
number  of  the  electrons  will  be  attracted  over  to  the  plate. 

This  action  takes  place  only  when  the  plate  is  positively 
charged  with  respect  to  the  filament;  if  the  connections  to  the 
battery  be  reversed  then  the  plate  will  be  negative  with 
respect  to  the  filament  and  repel  the  electrons  back  into  the 
filament  even  more  vigorously.  Thus  current  will  flow 
when  the  plate  is  positive  but  not  when  the  plate  is  negative. 
If  an  alternating  voltage  is  applied  between  plate  and  fila- 
ment, electrons  will  pass  and  a  current  flow  when  the  plate  is 
positive  but  no  action  takes  place  when  the  plate  is  negative. 
The  tube  acts  essentially  like  an  electrical  check  valve, 
allowing  current  to  pass  in  one  direction  but  shutting  it  out 
absolutely  in  the  other.  Just  how  this  rectifier  action  is 
utilized  in  the  receiving  circuit  will  be  discussed  in  a  later 
chapter. 

Space  Charge. — When  applying  comparatively  low  positive 
voltages  on  the  plate  we  find  experimentally  that  the  current 
obtained  represents  only  a  fraction  of  the  current  corre- 
sponding to  the  number  of  electrons  emitted  from  the  fila- 
ment. Thus  some  of  the  electrons  sent  off  from  the  filament 
must  have  been  attracted  back  into  the  filament  even  though 
the  plate  is  charged  positively.  This  action  is  attributed 
to  what  is  generally  known  as  "  space  charge."  Each  elec- 
tron carries  a  negative  charge,  similar  charges  repel  each 
other,  hence  there  is  always  a  mutual  repulsion  between 
electrons.  After  an  electron  has  left  the  filament  there  are 
two  forces  acting  upon  it,  one  the  force  of  attraction  of  the 
positively  charged  plate  and  the  other  a  force  of  repulsion 
due  to  the  electrons  which  have  left  the  filament  ahead  of  it 
and  which  on  account  of  the  repulsive  action  between  simi- 
larly charged  bodies  are  pushing  it  back  into  the  filament. 
Whether  the  electron  leaves  the  surface  of  the  filament  or 


18  ELEMENTS  OF  RADIO  TELEPHONY 

,,.,;L  ^  ,.(^-^,  ^  >-tK  ^^—A  '  / 
not  dppejlds  upon  whcthef  the  force  exerted  by  tne  plate  or 
that  clue  to  the  mutual  repulsion  action  is  the  stronger.  If 
the  first  predominates  it  will  pass  over  to  the  plate,  if  not 
it  will  be  repelled  back  into  the  filament.  Each  electron 
carries  the  same  charge,  hence  two  electrons  a  given  distance 
away  will  repel  another  electron  with  twice  the  force  that  a 
single  electron  would.  Thus  the  repulsive  effect  on  the 
electron  just  leaving  the  filament  depends  upon  how  many 
electrons  are  clustered  around  the  filament  in  their  motion 
toward  the  plate,  in  other  words  the  electron  density  in  the 
space  just  around  the  filament.  The  greater  the  density  of 
electrons  moving  away  from  the  filament  the  greater  the 
current  flowing  to  the  plate,  thus  for  any  given  positive  plate 
voltage  there  will  be  a  certain  electron  density  around  the 
filament  which  will  just  neutralize  the  pull  of  the  plate. 
This  prevents  any  increase  in  the  rate  at  which  electrons 
are  pulled  away  for  if  more  are  pulled  away  the  density 
around  the  filament  will  be  increased  and  the  electrons 
would  be  pushed  back  into  the  filament  with  more  force 
than  the  plate  exerts  to  pull  them  away.  If  the  voltage 
on  the  plate  is  increased  the  current  will  increase  to  the 
point  where  the  increased  electron  density  around  the  fila- 
ment is  again  sufficiently  great  to  nearly  balance  the  in- 
creased pull  of  the  higher  voltage  on  the  plate.  There  is  a 
limit  to  the  number  of  electrons  which  can  be  drawn  over  to 
the  plate,  however,  and  this  is  determined  by  the  total  num- 
ber of  electrons  emitted  by  the  filament  which  in  turn  is 
controlled  by  the  filament  temperature.  The  current  cor- 
responding to  a  complete  utilization  of  all  the  electrons  sent 
off  by  the  filament  is  generally  spoken  of  as  the  "  saturation 
current." 

Fig.  6  shows  how  the  current  flowing  to  the  plate  varies  as 
the  filament  current  is  maintained  at  a  constant  value  and 
the  plate  voltage  varied.  The  saturation  current  is  indi- 


VACUUM  TUBES 


19 


cated  by  the  sudden  flattening  out  of  the  curve.  This  indi- 
cates that  all  the  electrons  sent  off  from  the  filament  are 
being  utilized  and  hence  the  number  cannot  be  increased 
by  increasing  the  plate  voltage. 


FIG.  6. — Curve  showing  the  relation  between  plate  current  and  plate 
voltage.  The  dotted  curve  shows  the  effect  of  not  having  the 
filament  hot  enough. 


Grid  Electrode.— The  addition  of  the  third  electrode  to 
the  rectifying  valve  is  due  to  Dr.  Lee  DeForest,  who  first 
investigated  the  action  of  the  grid  electrode.  DeForest's 
three-element  tube  was  of  the  same  construction  as  the 
Fleming  valve  except  that  it  contained  a  third  electrode 
operating  cold  and  located  between  the  filament  and  plate. 


20  ELEMENTS  OF  RADIO  TELEPHONY 

The  third  electrode  enormously  enhances  the  value  of  the 
vacuum  valve  and  increases  its  functions  from  that  of  simple 
rectifier  to  amplifier,  amplifying  rectifier,  oscillator  and  mod- 
ulator. 

In  considering  the  action  of  the  third  electrode,  the  grid 
acts  merely  in  the  capacity  of  a  second  plate  in  so  far  as  the 


FIG.  7. — Three-electrode  vacuum  tubes.  Receiving  tube  on  the  left 
and  5-watt  transmitting  tube  on  the  right.  The  location  of  the 
plate,  grid  and  filament  is  indicated  by  the  letters  P,  G  and  F. 


electric  field  around  the  filament  is  concerned.  The  space 
charge  limitation  of  current  is  no  longer  determined  solely 
by  the  potential  of  the  plate  but  is  the  result  of  the  combina- 
tion of  the  potentials  of  both  plate  and  grid.  When  the  grid  is 
located  between  the  filament  and  plate  it  usually  has  a  much 
greater  relative  effect  on  the  limitation  of  the  electron  current 


VACUUM  TUBES  21 

by  space  charge  than  does  the  plate.  In  other  words  the  grid 
acts  like  a  second  plate  located  in  a  very  much  more  advan- 
tageous position.  In  some  large-sized  tubes  used  for  power 
purposes  this  ratio  of  effectiveness  runs  as  high  as  400, 
although  usually  much  less.  This  means  that  if  a  given 
decrease  in  plate  current  required  the  reduction  of  the  plate 
voltage  from  800  volts  to  400,  the  same  effect  could  have 
been  produced  by  lowering  the  potential  of  the  grid  by  one 
volt.  This  ratio  of  effectiveness  of  grid  and  plate  is  usually 
spoken  of  as  the  "  amplification  constant."  Grids  are  usually 
made  in  the  form  of  a  wire  network;  sometimes  wound  in  the 
form  of  a  helical  spring  with  the  filament  in  the  center  and 
with  plates  of  cylindrical  form  outside,  sometimes  built  in 
the  form  of  a  flat  gridiron  with  fine  wires  closely  spaced  for 
use  with  flat  plates.  The  amplification  constant  may  be 
increased  by  placing  the  grid  closer  to  the  filament  or  by 
decreasing  the  opening  between  grid  wires,  but  a  tube  with 
very  fine  grid  and  high  amplification  constant  gives  a  rela- 
tively smaller  plate  current  for  a  given  plate  voltage  since 
the  shielding  action  of  the  grid  makes  the  plate  voltage 
relatively  less  effective  in  drawing  the  electrons  across. 

Tungsten  and  Coated  Filaments. — Most  elements  require 
an  extremely  high  temperature  before  they  give  appreciable 
electron  emission,  hence  only  a  few  with  very  high  melting 
points  can  be  used  practically.  Tungsten  with  its  extremely 
high  melting  point  is  very  satisfactory  where  large  electron 
currents  are  required  and  is  used  extensively  for  both  trans- 
mitting and  receiving  tubes. 

It  has  been  found  that  numerous  chemical  compounds 
exhibit  the  same  thermionic  effects  as  pure  metals  and  in 
certain  case's  at  very  much  lower  temperatures.  The  oxides 
of  barium,  strontium  and  calcium  are  particularly  valuable 
in  this  respect  and  can  be  operated  at  a  fairly  low  red  heat  as 
compared  to  the  brilliant  white  heat  required  for  tungsten. 


22  ELEMENTS  OF  RADIO  TELEPHONY 

Since  the  power  input  to  the  filament  is  used  very  largely  to 
produce  heat  in  the  filament  and  the  heat  produced  in  the 
filament  is  necessarily  a  total  loss,  emission  at  low  tempera- 
tures represents  quite  a  saving  of  filament  power.  The 
usual  procedure  is  to  coat  a  platinum  wire  or  strip  with 


FICJ.  8. — Receiving  tuJje  (left)  and  5-watt  transmitting  tul>e  (right). 

certain  compounds  of  the  three  metals  which  change  into 
the  oxide  when  heated.  The  author  has  built  a  number  of 
tubes  using  other  materials  in  place  of  platinum  with  very 
good  success.  The  saving  in  power  for  a  given  filament 
emission  is  around  75  per  cent  as  compared  with  the  tung- 
sten filament  and  makes  it  almost  imperative  to  use  coated 


VACUUM  TUBES  23 

filaments   in   tubes    designed     to    be    operated   from    dry- 
cells. 

The  tubes  shown  in  Fig.  7  have  oxide-coated  platinum  fila- 
ments and  those  illustrated  in  Fig.  8  have  uncoated  tungsten 
filaments. 


CHAPTER   IV 
VACUUM  TUBE  OSCILLATORS 

The  ability  of  the  three-electrode  tube  to  produce  alter- 
nating currents  of  radio  frequencies  makes  it  almost  in- 
dispensable in  radio  communication  and  it  finds  application 
in  both  receiving  and  transmitting  circuits. 

The  production  of  alternating  currents  depends  upon  the 
fundamental  control  which  the  grid  has  upon  the  plate 
current.  This  effect  has  already  been  touched  upon  in  the 
previous  chapter  where  it  was  pointed  out  that  the  plate 
current  was  increased  or  decreased  as  the  grid  voltage  was 
made  more  positive  or  more  negative  with  respect  to  the 
filament.  The  grid  really  acts  like  an  electric  control  valve 
and  allows  more  or  less  energy  to  flow  from  the  plate  battery, 
and  since  under  proper  conditions,  the  grid  valve  can  be  made 
nearly  "  frictionless/'  electrically  speaking,  a  very  small 
amount  of  energy  applied  to  the  grid  may  control  a  large 
output  from  the  plate  battery.  Thus  it  can  be  seen  that 
this  device  should  have  a  distinct  application  in  amplifying 
weak  voltages. 

There  are  a  number  of  mechanical  illustrations  of  ampli- 
fier action,  for  instance,  the  tremendous  blow  of  the  steam 
hammer  can  be  controlled  by  an  easily  operated  valve  in  the 
hands  of  the  operator.  The  operator  presses  down  on  the 
control  valve  with  a  force  of  say  five  pounds  and  this  action 
controls  the  hammer  so  that  it  strikes  a  blow  of  five  tons  on 
the  anvil.  Then  he  may  operate  the  valve  in  the  other  direc- 
24 


VACUUM  TUBE  OSCILLATORS  25 

tion  with  another  five-pound  force  applied  upwards  and 
release  the  five-ton  pressure  on  the  anvil.  If  we  are  to 
consider  this  action  in  the  same  sense  of  amplifier  as  applied 
to  the  vacuum  tube,  we  would  say  that  the  five-pound 
"  blow  "  of  the  operator  had  been  amplified  into  a  five-ton 
blow  on  the  anvil.  In  other  words,  by  controlling  the  power 
supplied  from  the  boiler,  we  have  "  amplified  "  the  opera- 
tor's strength  two  thousand  times.  In  the  case  of  the  tube, 
the  grid  acts  like  the  steam  valve  which  allows  more  or  less 
of  the  boiler  pressure  (corresponding  to  the  voltage  of  the 
plate  battery)  to  act.  It  is  distinctly  not  a  device  from  which 
something  can  bo  obtained  for  nothing,  but  acts  merely  as  a 
control  of  some  secondary  source  of  energy. 

If  it  were  desirable,  we  could  make  the  steam  hammer 
self-acting  and  have  it  continue  to  move  up  and  down  auto- 
matically by  a  simple  arrangement  which  would  open  or  close 
the  valve  as  the  hammer  reached  the  end  of  its  stroke.  A 
possible  scheme  of  this  character  is  shown  in  Fig.  9.  The 
sliding  valve  shown  in  black  moves  up  and  down  on  its  seat. 
When  it  is  in  its  upper  position  it  uncovers  the  opening 
from  the  boiler  and  allows  steam  to  pass  into  the  lower  end 
of  the  cylinder,  where  it  acts  on  the  lower  side  of  the  piston 
and  pushes  it  upwards.  When  the  valve  is  in  the  lower  posi- 
tion, the  steam  from  the  boiler  is  cut  off,  and  a  passage  from 
the  cylinder  to  the  open  air  is  provided,  thus  allowing  the 
hammer  to  descend  from  its  own  weight.  In  order  to  make 
it  self-acting,  a  rod,  pivoted  at  the  center,  and  connected  to 
the  lower  end  of  the  valve  rod  is  so  arranged  that  it  is  moved 
when  the  hammer  reaches  either  end  of  its  stroke.  When  the 
hammer  hits  the  bottom  the  valve  rod  is  moved  upwards 
and  the  hammer  started  on  its  upstroke;  when  it  reaches  the 
top,  its  motion  is  reversed  and  the  hammer  falls.  This 
action  will  continue  until  the  steam  is  shut  off  at  the  boiler. 

One  important  requirement  for  continued  operation  is 


26 


ELEMENTS  OF  RADIO  TELEPHONY 


that  the  force  which  the  hammer  exerts  must  be  more  than 
that  necessary  to  open  and  close  the  valve;  in  other  words,  we 


hammer 


pipe  to 
toiler 


~$lide  valve 


waste  steam 


FIG.  9. — Steam  hammer  with  automatically  operated  control  valve. 


must  have  the  element  of  "  amplification  "  present  or  the 
apparatus  will  obviously  not  operate.  This  general  fact  is 
true  in  the  operation  of  any  self-controlled  system  of  this 


VACUUM  TUBE  OSCILLATORS 


27 


kind.  For  instance,  if  the  sound  produced  by  a  telephone 
receiver  is  of  greater  intensity  than  that  which  must  be 
impressed  on  the  transmitter  to  produce  the  same  sound, 
the  system  can  be  made  to  oscillate  and  produce  a  loud 
"  howl  "  by  placing  the  receiver  near  the  transmitter.  Here 
again  we  have  taken  a  portion  of  the  power  from  the  con- 
trolled circuit  and  put  enough  back  into  the  controlling  cir- 
cuit to  maintain  the  oscillation. 


input 
0.1  watt 

FIG.  lO.-Vacuum  tube  with  its  grid  excited  from  an  outside  power 
source. 


Feedback  Coupling.-In  applying  these  principles  to  the 
vacuum  tube,  we  must  take  some  of  the  energy  set  free 
from  the  plate  battery  and  bring  it  back  to  the  grid  cir- 
cuit where  it  can  in  turn  control  the  output  of  the  plate 
battery. 

Consider  Fig.  10  in  which  a  vacuum  tube  is  shown  with  its 
grid  circuit  connected  to  an  outside  source  of  power  As- 
sume that  it  takes  &  watt  input  to  completely  control  the 
output  of  the  plate  battery,  and  that  under  these  conditions 
the  plate  battery  can  deliver  10  watts  of  power.  If  the  grid 


28  ELEMENTS  OF  RADIO  TELEPHONY 

circuit  is  connected  to  a  60-cycle  source  of  power,  the  power 
output  of  the  plate  circuit  will  be  a  reproduction  of  the 
impressed  60-cycle  wave.  But  it  is  possible  to  take  TV  watt 
of  power  from  the  plate  circuit  and  still  have  a  possible  9.9 
watts  of  power  left.  Under  these  conditions  the  system 
could  be  made  to  oscillate  without  any  connection  to  an 
outside  source  by  taking  iV  watt  from  the  plate  circuit  and 
impressing  it  upon  the  grid. 

Frequency  of  Oscillation. — When  the  tube  connections 
are  changed  from  outside  excitation  to  self-excitation  by 
coupling  the  grid  circuit  over  on  to  the  plate  circuit,  the 
frequency  at  which  power  is  being  produced  in  the  plate 
circuit  will  probably  change  from  the  original  frequency  of 
60  cycles.  When  the  input  frequency  is  controlled  by  some 
outside  source  of  power,  the  tube  has  no  choice  but  to  repro- 
duce in  the  plate  circuit  what  has  been  impressed  on  the 
grid  circuit,  but  when  the  tube  is  self-excited  by  coupling 
plate  and  grid  circuits  together,  the  frequency  impressed  on 
the  grid  is  the  same  as  that  produced  in  the  plate  circuit. 
This  in  turn  is  controlled  by  the  grid  action  alone,  and  thus  a 
change  in  one  circuit  produces  a  complete  change  in  the  other. 
The  actual  outcome  is  that  after  a  very  short  period  of  time, 
the  tube  will  pick  out  the  frequency  at  which  it  can  most 
easily  oscillate  and  continue  to  operate  at  that  particular 
frequency.  This  frequency  will  be  determined  by  the 
electrical  constants  of  the  circuit,  and  is  analogous  to  the 
natural  frequencies  that  mechanically  vibrating  bodies  have. 
A  swing  will  swing  back  and  forth  very  easily  at  one  par- 
ticular speed,  but  if  an  attempt  is  made  to  speed  it  up  or 
slow  it  down,  it  will  require  considerable  exertion.  These 
critical  speeds  or  frequencies  are  generally  spoken  of  as 
"  resonant  "  or  "  natural  "  frequencies. 

Types  of  Feedback  Coupling.— There  are  a  number  of 
different  ways  in  which  a  portion  of  the  energy  from  the 


VACUUM  TUBE  OSCILLATORS 


29 


plate  circuit  may  be  fed  back  into  the  grid  circuit.  Fig.  11 
shows  a  method  in  which  the  changes  in  the  plate  current 
react  directly  on  the  grid  circuit  through  a  transformer.  As 
the  plate  current  in  coil  Lp  increases  and  decreases  it  sets 
up  alternating  voltages  in  Ly,  the  coil  connected  to  the  grid 
circuit,  and  thus  sustains  the  oscillations  in  the  plate  circuit. 
This  type  of  circuit  should  be  classed  under  the  head  of 
magnetically  coupled  circuits. 


Fiu.  11. — Magnetically  coupled 
feedback  circuit. 


FIG.  12.— Another  type  of  feedback 
circuit. 


Another  scheme  is  illustrated  in  Fig.  12.  Here  the  coil 
carrying  the  plate  current  and  the  grid  coil  have  no  mag- 
netic relation  to  one  another  but  are  merely  connected  in  the 
same  electrical  circuit.  When  a  pulsating  current  flows  in 
the  plate  circuit,  it  sets  up  an  alternating  voltage  in  the  plate 
coil  Lp.  The  plate  coil  being  a  part  of  the  heavy  lined 
oscillatory  circuit,  sets  up  a  current  in  this  circuit,  which  in 
turn  sets  up  a  voltage  across  the  grid  coil  L,.  Thus  the 


30 


ELEMENTS  OF  RADIO  TELEPHONY 


same  effect  is  produced  as  in  the  case  of  the  direct  magnetic 
coupling  just  described. 

The  third  system  illustrated  in  Fig.  13  is  essentially  the 
same  as  far  as  theoretical  relations  are  concerned  except 


FIG.  13. — Circuit  similar  to  Fig.  12  using  condensers  in  place  of  the 
inductance  coils. 


that  condensers  are  used  in  place  of  coils  with  essentially  the 
same  results. 

Electrostatic  coupling  is  illustrated  in  Fig.  14.  Here  the 
coupling  is  due  to  the  fact  that  the  condenser  C\  is  located 
in  the  part  of  the  circuit  common  to  both  grid  and  plate 
circuits.  The  alternating  component  of  the  plate  circuit 
passing  through  the  condenser  Ci  produces  a  voltage  drop 
across  C\.  Since  this  drop  is  also  in  the  grid  circuit,  its 


VACUUM  TUBE  OSCILLATORS 


31 


effect  is  carried  on  to  the  grid  and  oscillations  are  thus  sus- 
tained. The  direct  current  component  of  the  plate  current 
cannot  flow  through  the  condenser  so  a  bypath  around  the 
condenser  must  be  provided.  This  bypass  should  allow  the 
direct  current  to  flow  and  shut  out  the  alternating  current 


direct  current 
by-pass 


L 


L. 


9  ~f 

FIG.  14. — Electrostatically  coupled  feedback  circuit. 


so  that  it  has  to  pass  through  the  condenser.  This  function 
can  be  satisfactorily  performed  by  an  iron-cored  inductance 
coil  of  comparatively  low  resistance  and  connected  in  par- 
allel with  the  coupling  condenser.  Since  the  voltage  drop 
in  any  condenser  is  proportional  to  the  current  and  inversely 
proportional  to  the  product  of  the  frequency  and  the  capa- 
city of  the  condenser  for  a  given  high-frequency  current  in 


32  ELEMENTS  OF  RADIO  TELEPHONY 

the  plate  circuit,  the  voltage  impressed  in  the  grid  circuit 
will  be  inversely  proportional  to  the  capacity  of  the  con- 
denser. Thus  to  increase  the  effective  coupling  between 
grid  and  plate  circuits  the  condenser  capacity  should  be 
reduced.  Changing  the  capacity  of  the  coupling  condenser 
also  changes  conditions  in  the  plate  circuit  so  that  the 
relations  are  somewhat  complex. 

There  are  a  number  of  modifications  and  combinations 
of  the  above  circuits  applicable  to  practical  operation,  in  fact 
most  of  the  circuits  in  actual  use  employ  two  or  more  funda- 
mental types  of  coupling  simultaneously. 

Frequency. — In  predicting  the  frequency  of  oscillation  in 
circuits  similar  to  that  given  in  Fig.  12,  it  is  necessary  to 
know  the  inductance  of  the  coils  Lg  and  Lp  and  the  capacity 
of  the  condenser  C.  Where  these  values  are  known,  we  have 
the  relation 

300,000,000 


1884V(LP+L,)XC' 

where  /  is  the  frequency  in  cycles  per  second,  Le  and  Lp  are 
the  inductances  of  the  respective  coils  expressed  in  micro- 
henries and  C  the  capacity  of  the  condenser  expressed  in 
microfarads.  If  we  wish  to  express  the  results  in  terms 
of  wave  length  instead  of  frequency  the  following  relation 
would  be  used: 


wave  length  in  meters  =  1884V (Z,,+L;, 

In  generating  very  short  waves  where  the  capacity  of  the 
condenser  C  is  very  low,  the  internal  capacity  of  the  tube 
and  tube  socket  must  be  taken  into  account.  In  one  type  of 
receiving  circuit,  formerly  very  popular  with  the  amateurs, 
the  only  capacity  employed  was  that  of  the  tube  and  the 


VACUUM  TUBE  OSCILLATORS  33 

socket  together  with  the  capacity  effects  of  the  coils  them- 
selves, with  the  condenser  C  omitted  entirely. 

If  there  is  any  magnetic  coupling  between  the  two  coils 
another  term  must  be  added  to  the  above  equations.  The 
quantity  in  parenthesis  will  now  be  increased  by  2M,  where 
M  is  the  mutual  inductance  of  the  two  coils. 

The  circuit  shown  in  Fig.  13  can  be  solved  in  the  same 
way  if  we  substitute  the  equivalent  capacity  of  the  two  con- 
densers for  the  value  of  C  as  given  in  the  equation.  The 
expression  for  the  equivalent  value  of  two  condensers  con- 
nected in  series  is 


Plate-voltage  Supply  Systems. — The  principal  problem  in 
the  operation  of  power  tubes  is  the  high-voltage  plate  supply, 
and  it  may  be  solved  in  several  different  ways.  The  poten- 
tial required  varies  with  the  size  of  the  tube  used.  For 
standard  small  power  tubes  rated  at  5-watts  output,  this 
value  lies  around  300  volts;  for  tubes  of  50-watts  output,  it  is 
1000  volts;  for  250  watts,  it  is  2000  volts;  and  up  to  20,000 
volts  for  higher  powered  tubes.  Standard  2000-volt  direct- 
current  generators  are  now  available  but  above  this  value 
it  is  usual  to  use  a  rectifier  system  in  connection  with  an 
alternating-current  supply,  the  resulting  output  being 
smoothed  out  with  suitable  electrical  filters  consisting  of 
condensers  and  inductance  coils. 

Direct-current  Generators. — The  construction  of  direct- 
current  generators  up  to  500  volts  is  so  well  known  that  only 
certain  special  types  of  machines  will  be  considered  here. 
One  requirement  of  all  plate-voltage  generators  is  that  the 
voltage  should  be  as  constant  and  as  free  from  ripples  as 


34  ELEMENTS  OF  RADIO  TELEPHONY 

possible.  This  requirement  should  be  met  by  having  as 
many  commutator  segments  as  possible  since  the  disturb- 
ance caused  by  the  brushes  passing  from  one  segment  to  the 
next  may  be  thus  reduced  in  the  inverse  proportion  of  the 
number  of  commutator  segments.  High-voltage  generators 
usually  require  a  large  number  of  commutator  segments  for 
insulational  purposes  also.  For  sets  which  are  to  be  sup- 
plied from  storage  batteries  a  special  type  of  combination 
motor  and  generator  has  been  developed.  This  machine, 


FIG.  15. — Dynamotor  for  producing  high-voltage  direct  current  from  a 
low-voltage  direct-current  supply. 


which  is  known  as  a  "  dynamotor,"  has  a  single  field  and 
armature,  but  two  separate  windings  on  the  armature  which 
are  insulated  from  one  another.  Each  of  these  windings  is 
brought  out  to  its  own  commutator,  one  of  which  is  located 
on  each  end  of  the  shaft.  The  low-voltage  winding  designed 
for  a  value  around  10  or  12  volts  is  connected  to  the 
storage  battery  and  acts  as  the  motor,  while  the  other  wind- 
ing produces  a  high  potential  of  about  350  volts  for  the  plate 
supply.  The  field  winding  is  supplied  from  the  storage 
battery.  A  similar  piece  of  apparatus  is  sometimes  used  in 


VACUUM  TUBE  OSCILLATORS 


35 


airplane  sets,  the  machine  in  this  case  acts  as  a  double- 
current  generator  driven  mechanically  by  an  air  fan  and 
supplies  both  plate  and  filament  circuits  simultaneously. 

For  voltages  up  to  1000  volts  a  single-commutator  machine 
is  usually  employed,  above  this  value,  double-commutator 
machines  are  preferable.  The  standard  2000-volt  generator 


FIG.  16. — Rectifier  connections  in  which  but  one-half  of  the  alternating, 
current  wave  is  utilized. 


built  by  one  manufacturer  has  two  1000-volt  windings : 
brought  out  to  separate  commutators  on  each  end  of  the. 
shaft.  The, two  windings  are  connected  in  series  to  produce 
2000  volts.  The  field  excitation  on  this  machine  is  prefer-, 
ably  arranged  to  be  furnished  from  a  separate  low-voltage 
supply,  thus  keeping  the  field  circuit  entirely  insulated  from  • 
the  high-voltage  windings. 


36 


ELEMENTS  OF  RADIO  TELEPHONY 


Rectifier  Systems.— Rectifier  systems  are  applicable  from 
about  500  volts  upwards.  Although  they  may  be  used 
below  500  volts,  the  difficulties  involved  in  smoothing  out 
the  wave  usually  make  the  generator  preferable. 

High-powered  tubes  require  high-voltage  and  low-current 
supply,  hence  rectifiers  utilizing  pure  electron  emission  are 


high  vofrage 
cftrecf  current 


0000000 
/ow  vo/fage 
affernaf/rtg 


FIG.  17.—  Rectifier  utilizing  both  halves  of  the  alternating  current  wavo. 

better  suited  than  those  of  the  mercury  arc  type.  As  com- 
mercially manufactured  they  much  resemble  the  power 
tubes  which  they  are  designed  to  supply  except  that  the  grid 
being  unnecessary  is  of  course  omitted.  Fig.  16  shows  the 
connections  employed  where  one  tube  is  used  and  only  one- 
half  of  the  wave  rectified.  Double-wave  rectification  em- 
ploying two  tubes  and  utilizing  both  halves  of  the  wave  is 
much  to  be  preferred  in  the  case  of  radio  telephone  trans- 


VACUUM  TUBE  OSCILLATORS 


37 


mitters.  The  currents  in  the  various  portions  of  the  circuit 
indicated  in  Fig.  17  are  illustrated  in  Fig.  18.  The  con- 
denser Ci  acts  as  an  electrical  storage  reservoir  for  the 
energy  which  passes  into  it  in  impulses  as  delivered  from  the 
rectifier.  The  irregularities  are  further  eliminated  by 
the  series  inductance  coils,  L\  and  Z>2,  which  are  composed 


current  in    (T\ 


AAAA/L 

current  in© 


current  in  (3 


FIG.  18. — Voltage  and  rectified  currents  as  indicated  in  Fig.-  17. 

of  a  very  large  number  of  turns  wound  on  an  iron  core.  The 
energy  is  transferred  to  the  condenser  Cs  which  acts  as 
another  storage  reservoir  and  serves  to  still  further  smooth 
out  the  variations.  Coil  LI  is  shunted  by  another  condenser 
€2,  and  the  two  values  are  adjusted  so  that  they  are  resonant 
together  at  the  supply  frequency.  This,  as  can  be  math- 
matically  shown,  produces  a  very  high  resistance  to  the 
passage  of  currents  of  this  particular  frequency  and  is  an 


38 


ELEMENTS  OF  RADIO   TELEPHONE 


additional  aid  to  the  elimination  of  the  low-frequency  ripple. 
Since  irregular  waves  are  composed  of  a  large  number  of 
multiple-frequency  waves  the  single-frequency  trap  cannot 


Fia.  19. — Hot  cathode  or  "Kenotron"  rectifier, 
ampere  at  100,000  volts. 


Normal  rating  0.1 


eliminate  all  of  the  disturbing  variation.  If  it  is  desired 
to  smooth  out  the  wave  still  further,  other  sections  of  series 
inductance  and  shunt  capacity  may  be  added  with  increas- 
ingly better  results.  With  properly  designed  filter  systems 


VACUUM  TUBE  OSCILLATORS  39 

direct-current  voltages  may  be  produced  which  compare 
very  favorably  with  those  produced  by  direct-current  gen- 
erators, and  which  are  very  well  suited  for  radio  telephony. 

Cold-cathode  Rectifiers. — A  cold-cathode  rectifier  has 
recently  been  developed  by  one  manufacturer.  This  appa- 
ratus consists  of  two  electrodes  mounted  inside  a  glass  bulb 
in  a  much-reduced  atmosphere  of  certain  gases.  Its  opera- 
tion hinges  on  the  peculiar  effects  of  ionization  by  collision 
as  experienced  in  the  action  of  soft  detector  tubes  under 
which  heading  this  action  is  treated  in  some  detail.  The 
electrodes  are  so  arranged  that  all  electrons  passing  from 
one  of  the  electrodes  to  the  other  are  absorbed  before  any 
ionization  by  collision  with  the  gas  particles  can  be  effected 
while  electrons  passing  in  the  other  direction  have  a  long 
enough  path  so  that  they  can  produce  new  electrons  and 
positive  ions  by  collision.  The  result  is  that  when  potential 
is  applied  in  one  direction  a  very  minute  current  consisting 
of  the  free  electrons  in  the  gas  will  flow,  but  on  reversing  the 
direction  of  the  voltage  the  current  will  be  very  much 
increased  due  to  the  additional  electrons  and  positive  ions 
set  free  by  collision.  The  rectification  is  not  perfect  since 
some  current  will  flow  in  either  direction  but  the  reversed 
current  is  very  small  and  almost  negligible. 

The  connections  used  in  its  operation  are  exactly  the  same 
as  those  shown  for  the  hot-cathode  rectifier  except  that  the 
circuit  used  for  lighting  the  filament  is  not  needed. 

Electrolytic  Rectifiers. — On  account  of  the  extreme  sim- 
plicity of  construction  the  electrolytic  rectifier  is  quite  well 
suited  for  the  rectification  of  small  amounts  of  alternating- 
current  power  at  moderate  voltages.  It  consists  of  active 
plates  of  aluminum  and  inactive  plates  of  carbon  or  lead 
immersed  in  electrolyte.  There  are  a  number  of  different 
solutions  which  can  be  used  but  a  saturated  water  solution 
of  pure  boracic  acid  seems  to  be  a  favorite.  The  voltage 


40 


ELEMENTS  OF  RADIO  TELEPHONY 


that  each  cell  will  take  care  of  is  dependent  upon  the  con- 
struction but  usually  does  not  exceed  200  volts,  hence 
a  number  of  cells  must  be  connected  in  series  where  voltages 
above  this  value  are  used.  The  connections  used  are 


FIG.  20. — Cold  cathode  rectifier  and  electrolytic  condenser. 


exactly  similar  to  those  shown  for  use  with  hot-cathode  rec- 
tifiers with  the  vacuum  tubes  replaced  by  the  proper  number 
of  cells  connected  in  series.  The  filament  circuit  is  of  course 
not  needed.  Obviously  either  single-  or  double-wave  rec- 
tification may  be  obtained  by  the  proper  connection. 


VACUUM  TUBE  OSCILLATORS  41 

Proper  operation  is  indicated  by  a  series  of  star-like  sparks 
at  the  surface  of  the  aluminum  electrode  and  care  should  be 
taken  to  provide  sufficient  capacity  so  that  the  electrolyte 
does  not  become  very  hot  as  rectification  may  be  partially 
or  totally  prevented  by  high  temperature. 

Filter  Condensers. — Filter  condensers  must  have  '  high 
capacity  and~be  able  to  withstand  the  full-plate  voltage. 
These  conditions  are  best  fulfilled  by  mica  or  treated  paper 
condensers  since  air  condensers  are  veiy  bulky  in  large 
capacities.  Ordinary  paraffined  paper  telephone  condens- 
ers are  suitable  if  the  plate  voltage  does  not  exceed  300 
volts,  but  where  voltages  go  much  above  this  value 
mica  or  specially  treated  paper  condensers  should  be  used. 
It  is  not  safe  to  connect  two  or  more  condensers  in  series 
across  a  high-voltage  direct-current  supply  as  the  voltage 
will  divide  between  the  various  units  in  direct  proportion 
to  their  individual  insulation  resistances.  Thus  if  two 
condensers  are  used  in  series  and  one  is  slightly  "  leaky  "  the 
full  voltage  will  be  thrown  across  the  good  one  and  will 
usually  puncture  it.  It  is  possible  to  equalize  the  voltages 
in  such  cases  by  connecting  equal  resistances  across  each 
condenser,  but  this  is  wasteful  of  energy. 

Filter  condensers  are  usually  not  designed  to  carry  radio- 
frequency  currents  since  large  losses  are  induced  by  the  high- 
frequency  which  heats  the  condenser  internally  and  eventu- 
ally destroys  its  insulation.  Where  there  is  any  possibility 
of  this  happening  a  radio-frequency  choke  and  by-pass 
condenser  capable  of  handling  radio-frequency  current  must 
be  provided.  The  by-pass  condenser  need  not  be  very  large 
but  must  be  capable  of  standing  up  under  the  full  plate 
voltage.  Its  capacity  need  not  be  more  than  .001  mf.  for 
short  waves.  The  radio-frequency  choke  will  vary  in  size 
depending  upon  the  wave  length  used.  For  amateur  waves 
around  200  meters  a  coil  consisting  of  from  100  to  200  turns 


42  ELEMENTS  OF  RADIO   TELEPHONY 

of  wire  wound  on  a  tube  about  2 \  inches  in  diameter  in  a 
single  layer  is  sufficient.  The  location  of  the  choke  should 
be  between  the  last  filter  condenser  and  the  oscillator  tube 
and  the  by-pass  condenser  should  be  located  on  the  oscillator 
side  of  the  choke. 

Electrolytic  Condensers.  —  Electrolytic  condensers  are 
formed  by  immersing  aluminum  electrodes  in  certain  solu- 
tions and  passing  a  direct  current  between  them.  The 
action  of  the  electric  current  is  to  decompose  some  of  the 
electrolyte  into  gas  and  form  a  thin  film  of  gas  between  the 
surface  of  the  plate  and  the  electrolyte.  Since  the  gas  film  is 
extremely  thin  the  capacity  per  unit  area  is  very  high  and 
large  capacity  condensers  can  be  easily  constructed  by  pro- 
viding fairly  large  electrode  surface.  The  breakdown  volt- 
age of  this  type  of  condenser  depends  upon  the  electrolyte 
used,  one  authority  gives  480  volts  for  sodium  borate,  470 
volts  for  ammonium  citrate,  460  volts  for  ammonium  acid 
phosphate  and  445  volts  for  sodium  silicate.  One  advantage 
of  this  type  of  condenser  is  that  it  is  self-healing;  if  it  is  punc- 
tured, reduce  the  voltage  and  build  up  a  new  gas  film  and 
it  will  function  as  well  as  ever. 

The  electrolytic  condenser  illustrated  has  special  aluminum 
electrodes  designed  to  give  very  large  area  in  contact  with 
the  electrolyte  and  has  very  high  capacity.  The  capacity 
is  so  high  that  in  certain  cases  the  filter  inductance  may  be 
greatly  decreased  in  size  or  entirely  omitted  with  very  satis- 
factory results. 

Radio-frequency  Power  Amplifiers. — On  account  of  the 
very  sharp  tuning  possible  with  tube  transmitters  it  is  very 
important  that  the  transmitted  frequency  be  maintained 
very  nearly  constant.  In  circuits  where  the  tube  supplies  its 
own  excitation  by  coupling  back  from  the  plate  circuit, 
any  change  in  the  constants  of  the  plate  circuit  or  circuits 
associated  with  the  plate  circuit  will  have  a  corresponding 


VACUUM  TUBE  OSCILLATORS 


43 


effect  on  the  frequency  of  oscillation.  Since  in  most  circuits 
the  antenna  is  either  directly  or  inductively  coupled  to  the 
plate  circuit,  any  change  in  the  antenna  inductance  or  capac- 
ity will  react  back  upon  the  plate  circuit  and  produce  a 
very  annoying  change  in  frequency.  Unless  the  antenna 
is  very  well  guyed,  it  will  swing  in  the  wind,  and  this  motion 
will  change  its  effective  capacity.  This  will  in  turn  produce 


From 
master 
oscillator. 


FIG.  21. — Master  oscillator  and  radio  frequency  power  amplifier. 


a  change  in  the  transmitted  wave  length.  To  eliminate  this 
difficulty,  the  circuit  shown  in  Fig.  21  has  been  devised. 
The  coil  L\  is  connected  to  the  output  circuit  of  the  master 
oscillator  tube  which  for  radio  telephony  would  be  arranged 
for  modulation  by  some  one  of  the  modulation  systems  out- 
lined elsewhere.  The  output  of  the  master  oscillator  is 
impressed  on  the  grid  of  the  power  amplifier  tube  and  is  in 
turn  amplified  and  put  into  the  antenna  circuit.  Since 


44  ELEMENTS  OF  RADIO  TELEPHONY 

there  is  no  connection  between  the  antenna  and  the  master 
oscillator  tube  except  back  through  the  amplifier  which  only 
operates  in  one  direction,  a  variation  in  the  antenna  constants 
will  not  affect  the  frequency  of  oscillation.  The  power 
rating  of  the  master  oscillator  may  be  very  much  lower  than 
the  amplifier,  or  one  master  oscillator  tube  may  control 
several  tubes  of  the  same  type  connected  together  as  ampli- 
fiers. The  grids,  filaments  and  plates  of  the  several  amplifiers 
would  be  respectively  connected  together  and  should  operate 
satisfactorily  if  their  construction  is  uniform. 

Transmitting  Tube  Circuits. — For  the  sake  of  simplicity 
the  plate-voltage  supply  has  been  shown  in  nearly  all  dia- 
grams as  connected  next  to  the  plate  in  the  plate  circuit. 
While  this  is  satisfactory  in  the  case  of  receiving  tubes  it  is 
not  desirable  in  the  case  of  power  tubes  as  it  puts  the  gene- 
rator or  rectifier  system  in  a  high  potential  part  of  the 
circuit  as  far  as  radio-frequency  voltage  is  concerned.  This 
may  induce  considerable  strain  between  the  windings  and 
the  ground  and  may  puncture  the  insulation.  In  order  to 
avoid  this  condition  the  proper  position  for  the  generator 
or  rectifier  system  is  next  to  the  filament  circuit  with  the 
negative  side  connected  directly  to  the  filament.  If  the  fila- 
ment is  supplied  with  alternating  current  this  connection 
should  be  made  to  the  center  tap  on  the  filament  winding. 
This  common  point  is  usually  grounded  to  guard  against 
the  possibility  of  any  considerable  potential  being  set  up 
between  generator  windings  and  frame.  These  precautions 
have  been  indicated  in  Fig.  21. 

In  nearly  all  c:rcuits  shown  the  plate-voltage  supply  has 
been  connected  in  series  with  the  plate  coil.  It  may  also  be 
connected  in  parallel  with  the  plate  coil  if  a  condenser  is 
inserted  in  series  with  the  plate  coil  to  prevent  it  short-cir- 
cuiting the  plate-voltage  supply.  In  order  to  prevent  the 
high-frequency  current  from  passing  through  the  generator 


VACUUM  TUBE  OSCILLATORS 


45 


rather  than  through  the  plate  coil  the  by-pass  condenser 
across  the  generator  terminals  should  be  omitted  and  a  radio- 
frequency  choke  inserted  in  one  or  both  leads. 

There  are  two  general  types  of  possible  connection  with 
the  antenna  circuit,  inductive  and  direct. 


IS 


FIG.  22. — Power  tubes  rated  at  50,  250,  and  5  watts  output  respectively. 


Inductive  coupling  is  illustrated  in  Fig.  21  by  the  solid 
lined  antenna  circuit.  This  system  offers  advantages  where 
the  antenna  system  is  of  high  resistance  or  where  very  short 
wave-length  transmission  is  desired. 


46  ELEMENTS  OF  RADIO  TELEPHONY 

Direct  coupling  is  obtained  by  completing  the  antenna 
circuit  through  several  turns  of  the  plate  coil.  This  type  of 
connection  is  illustrated  in  Fig.  21  by  the  dotted  antenna 
circuit.  The  number  of  turns  used  should  be  adjustable  to 
compensate  for  different  antenna  and  tube  characteristics. 
The  condenser  in  the  ground  lead  is  necessary  to  prevent  the 
plate-voltage  generator  being  short-circuited  through  the 
double-ground  connection.  With  a  high-resistance  antenna 
and  direct  coupling  it  is  sometimes  difficult  to  make  the 
tube  oscillate  properly  on  account  of  the  large  resistance 
effect  of  the  antenna.  In  such  cases  it  is  advisable  to  employ 
inductive  coupling  as  the  adjustments  are  somewhat  simpler. 


CHAPTER  V 
MODULATOR  SYSTEMS 

The  modulator  in  radio  transmission  has  the  same  duty  as 
the  transmitter  does  in  wire  transmission.  If  the  transmitter 
does  not  alter  its  resistance  the  value  of  the  direct  current 
flowing  through  the  circuit  will  remain  unchanged  and  the 
telephone  receiver  diaphragm  will  not  vibrate;  only  when 
the  current  is  changing  in  value  is  any  sound  transmitted 
and  the  modulator  in  radio  telephony  is  the  device  that 
varies  the  output  of  the  set  in  accordance  with  the  sound 
wave  that  it  is  desired  to  transmit. 

There  are  two  general  schemes  which  are  employed  in 
radio  telephony,  in  the  first  one  the  tube  delivers  a  constant 
power  output  and  the  transmitted  wave  is  varied  in  ampli- 
tude by  changing  conditions  in  the  antenna  circuit;  the 
second  one  does  not  vary  antenna  conditions  but  controls 
the  output  of  the  tube  in  accordance  with  the  sound  wave  to 
be  transmitted. 

Modulation  by  Antenna  Control. — The  simplest  case  of 
modulation  by  antenna  control  is  illustrated  in  the  use  of 
an  ordinary  telephone  transmitter  or  microphone  connected 
directly  in  the  antenna  circuit.  The  telephone  transmitter, 
as  has  been  pointed  out,  is  a  device  which  changes  its  resist- 
ance to  correspond  with  the  sound  waves  impressed  upon  its 
diaphragm,  hence  the  transmitter  connected  directly  in 
series  with  the  antenna  will  cause  the  current  to  rise  and  fall 
as  the  resistance  decreases  and  increases.  The  one  serious 
47 


48  ELEMENTS  OF  RADIO  TELEPHONY 

limitation  which  this  system  involves  is  the  small  amount  of 
power  which  can  be  successfully  handled  by  its  use.  Where 
the  antenna  current  is  only  a  small  fraction  of  an  ampere, 
the  ordinary  telephone  transmitter  will  act  satisfactorily, 
and  where  larger  currents  are  involved  special  high-current 
microphones  have  been  developed,  but  the  system  as  a 
whole  is  wasteful  of  power  and  not  satisfactory. 

Magnetic  Modulator. — Another  more  satisfactory  solu- 
tion involves  the  use  of  the  magnetic  modulator.  A  sche- 
matic drawing  of  the  magnetic  modulator  is  shown  in  Fig.  23. 
The  iron  core  is  composed  of  a  strip  of  very  thin  transformer 
iron  wound  into  the  form  of  a  hollow  cylinder.  The  direct- 
current  winding,  which  is  wound  through  the  center  of  the 
cylinder,  is  connected  in  series  with  a  battery  and  telephone 
transmitter.  The  high-frequency  current  winding  is  placed 
around  the  outside  of  the  cylinder  at  right  angles  to  the 
direct-current  winding.  For  short  waves  this  winding  is 
wound  in  two  parts  and  connected  in  series,  the  two  halves 
being  wound  in  opposite  directions.  The  high-frequency 
conductor  is  composed  of  a  number  of  fine  insulated  copper 
wires  braided  together,  commonly  called  "  litzendraht."  , 
This  type  of  wire  has  a  much  lower  resistance  at  high  fre"-  < 
quencies  than  solid  wire.  The  iron  core  is  of  special  thin 
high-frequency  iron  and  carefully  insulated. 

The  action  of  this  type  of  modulator  is  difficult  to  explain 
without  recourse  to  rather  advanced  mathematics,  briefly 
the  action  depends  upon  the  effect  that  varying  magnetic 
saturation  has  upon  the  modulator.  When  the  direct  cur- 
rent from  the  microphone  is  large  the  iron  becomes  nearly 
saturated,  that  is,  it  will  only  slightly  respond  to  further 
increases  in  current.  This  changes  the  characteristics  of 
the  high-frequency  coil  also  and  hence  the  current  in  the 
antenna.  This  effect  is  varied  to  a  greater  or  less  degree 
depending  upon  the  value  of  current  flowing  through  the 


MODULATOR  SYSTEMS 


49 


direct-current  winding,  which  is  in  turn  controlled  by  the 
resistance  of  the  microphone,  and  thus  the  sound  waves  are 
reproduced  in  the  amplitude  of  the  antenna  current. 


sheef-  iron  core 


FIG.   23. — Magnetic  modulator  showing  connections  to    power  tube 
oscillator  and  antenna. 


The  current-carrying  capacity  of  this  apparatus  is  not 
limited  as  was  the  case  with  the  microphone  and  it  can  be 
employed  to  modulate  very  large  outputs.  For  simplicity  of 
operation  it  can  hardly  be  unproved  upon,  and  the  results 
obtained  with  its  use  are  very  satisfactory. 


50 


ELEMENTS  OF  RADIO  TELEPHONY 


Modulation  by  Variation  of  Tube  Output. — This  system 
differs  from  the  one  just  described  in  that  the  constants  of 
the  antenna  circuit  remain  unchanged  and  the  output  of  the 
tube  oscillator  is  made  to  vary  in  accordance  with  the  sound 
wave  to  be  transmitted.  There  are  two  general  ways  in 
which  the  output  of  the  tube  oscillator  may  be  varied :  (a)  by 


FIG.  24. — Plate  circuit  modulator  connections. 


variation  of  the  plate  voltage  and  (6)  by  variation  of  the 
grid  voltage. 

Plate-circuit  Modulation. — Connections  for  plate-circuit 
modulation  are  shown  in  Fig.  24.  The  terminals  A  and  B 
are  connected  to  a  tube  oscillator  in  place  of  the  usual  high- 
voltage  supply  connections.  P  and  S  are  two  coils  of  an 
iron-cored  transformer,  the  plate  current  to  the  oscillator 
tube  flowing  through  S,  and  the  plate  current  of  the  modu- 


MODULATOR  SYSTEMS  51 

lator  tube  flowing  through  P.  If  the  plate  current  from  the 
modulator  tube  remains  constant  no  voltage  will  be  induced 
in  the  coil  S,  and  the  voltage  applied  at  the  plate  of  the  oscil- 
lator tube  will  be  that  generated  by  the  direct-current  genera- 
tor G.  Any  variation  in  the  current  flowing  through  P  will 
set  up  a  voltage  hi  coil  S,  and  since  this  coil  is  in  series  with 
the  plate  of  the  oscillator  tube,  the  actual  voltage  delivered 
to  the  oscillator  tube  will  be  the  sum  of  the  voltage  produced 
by  the  generator  and  that  set  up  in  the  coil  S  due  to  the 
change  of  current  in  coil  P.  Sometimes  this  voltage  will  be 
in  such  a  direction  as  to  add  to  the  voltage  supplied  by 
the  direct-current  generator  and  at  other  times  it  will  oppose 
the  generator  voltage  and  lower  its  effective  value.  The 
output  of  the  oscillator  is  thus  changed,  its  value  being  low 
when  the  applied  voltage  is  low  and  large  when  the  applied 
voltage  is  high. 

When  sound  waves  impinge  upon  the  diaphragm  of  the 
microphone  M,  its  resistance  is  changed  and  the  current 
flowing  through  the  primary  coil  PI  of  the  modulation  trans- 
former is  changed  in  accordance  with  the  resistance  varia- 
tion. This  sets  a  voltage  up  in  the  secondary  coil  Si  which 
in  turn  is  transmitted  to  the  grid  of  the  modulator  tube. 
Variation  of  the  grid  voltage  varies  the  plate  current,  and 
finally  changes  the  voltage  applied  to  the  plate  of  the  oscil- 
lator tube  thus  changing  its  output. 

It  is  not  necessary  to  use  two  separate  coils  P  and  S,  as 
both  plate  currents  may  be  carried  through  a  single  coil. 
This  corresponds  to  the  "  auto-transformer "  connection 
well  known  in  electrical  work  and  is  essentially  the  same  as 
far  as  electrical  characteristics  are  concerned  as  the  two- 
coil  transformer  with  equal  number  of  turns.  The  connec- 
tion employing  the  single  coil  has  been  erroneously  termed 
the  "  constant  current  "  method  of  modulation,  the  assump- 
tion being  that  the  current  in  the  single  coil  has  a  single 


52  ELEMENTS  OF  RADIO  TELEPHONY 

constant  value.  The  error  in  this  assumption  can  easily  be 
proven  from  a  simple  mathematical  treatment  of  the  circuit. 
When  this  system  is  in  proper  adjustment,  the  modulation 
is  extremely  good  with  the  additional  advantage  of  controlling 
the  entire  antenna  output  rather  than  only  a  small  portion 
as  is  the  case  with  some  other  systems.  When  once  properly 
adjusted,  it  will  operate  satisfactorily  over  a  wide  variation 
of  operating  conditions  without  further  adjustment.  It  has 
the  disadvantage  of  requiring  at  least  two  tubes  of  approx- 
imately equal  output,  and  the  output  of  only  one  tube  is 
recoverable  as  high-frequency  power  for  signal  production. 
Two  tubes  used  together  as  oscillators  and  modulated  with  a 
microphone  in  the  antenna  circuit  will  be  required  to  produce 
theoretically  the  same  signal  when  properly  adjusted  as  one 
tube  acting  as  oscillator  and  one  as  modulator. 

Grid  Modulation. — In  the  general  consideration  of  vacuum 
tubes  it  was  pointed  out  that  the  current  in  the  plate  circuit 
could  be  controlled  in  two  different  ways,  by  varying  the 
plate  voltage  a  given  amount  or  by  varying  the  grid  voltage 
by  a  correspondingly  much  smaller  amount.  Thus  the  out- 
put of  an  oscillator  can  be  controlled  by  the  introduction  of  a 
variable  voltage  in  the  plate  circuit  or  by  a  similar  voltage  of 
much  smaller  value  introduced  into  the  grid  circuit.  Fig.  25 
illustrates  a  possible  connection  for  grid  modulation.  The 
microphone  is  connected  in  series  with  a  battery  and  the 
primary  coil  of  the  modulation  transformer  and  the  secondary 
voltage  instead  of  being  impressed  on  the  grid  of  the  modu- 
lator tube  is  put  into  the  grid  circuit  of  the  oscillator  tube 
directly.  A  condenser  is  connected  across  the  terminals  of 
the  secondary  of  the  transformer  in  order  to  allow  the  high- 
frequency  currents  to  pass  which  would  have  otherwise  been 
shut  off  by  the  inductance  of  the  secondary  winding.  This 
condenser  can  also  act  as  the  grid  condenser  in  the  oscil- 
lating circuit,  and  the  resistance  of  the  secondary  coil  being 


MODULATOR  SYSTEMS 


53 


very  high  may  serve  as  the  grid  leak  resistance.  This  system 
is  quite  difficult  to  adjust  and  maintain  in  operating  condi- 
tion, and  the  results  are  in  general  much  inferior  to  plate 
circuit  modulation.  It  is  difficult  to  modulate  any  consid- 
erable proportion  of  the  output  without  getting  into  diffi- 


modulation 
Transformer 


to  antenna  - — /~G~5~6~S~0~>| 


FIQ.  25. — Transmitting  tube  arranged  for  grid  circuit  modulation. 


culty  due  to  the  tube  stopping  oscillation  entirely.  When 
the  voltage  of  the  secondary  of  the  modulation  transformer 
makes  the  grid  negative,  the  output  is  reduced  but  not  pro- 
portionately. There  seems  to  be  a  point  of  instability  which 
when  reached  stops  the  tube  oscillation  instead  of  merely 
reducing  it.  This  causes  very  poor  modulation  and  is 
difficult  to  avoid  if  complete  modulation  is  desired. 


54  ELEMENTS  OF  RADIO  TELEPHONY 

The  value  of  the  condenser  Cg  plays  an  important  part  in 
the  proper  adjustment  and  should  be  varied  by  trial  for  best 
results.  It  is  difficult  to  maintain  this  system  in  adjustment 
as  variations  of  plate  voltage,  filament  current,  antenna 
constants,  etc.,  all  seem  to  require  compensating  readjust- 
ments. 


CHAPTER  VI 
RECEIVING  EQUIPMENT 

Tuning. — If  one  will  depress  the  "  loud  "  or  damper  pedal 
on  a  piano  and  sing  some  tone  of  constant  pitch  while  the 
pedal  is  down  the  set  of  strings  corresponding  to  that  pitch 
will  be  set  into  vibration.  This  fact  can  be  verified  by  sud- 
denly stopping  the  tone  and  listening  intently  while  the 
pedal  is  still  held  down  when  the  string  vibration  can  be 
recognized  by  a  faint  ringing  tone  produced  by  the  vibrating 
strings.  This  phenomenon,  which  is  known  as  "  sympathetic 
vibration,"  is  the  basis  of  all  radio  tuning.  The  transmitter 
sets  up  a  note  in  the  ether  corresponding  to  the  sound  pro- 
duced vocally  and  this  radio  "  note  "  travels  out  in  all  direc- 
tions and  impinges  on  hundreds  of  antenna  systems  which 
correspond  to  the  strings  on  the  piano,  but  the  only  ones  on 
which  it  has  any  appreciable  effect  are  those  which  are 
tuned  to  the  same  frequency  or  wave  length  as  the  trans- 
mitted wave.  To  produce  different  tones  on  the  piano  we 
tighten  or  loosen  the  strings  which  are  of  light  weight  in  the 
upper  register  and  heavy  in  the  bass  section;  in  radio  tuning 
we  accomplish  the  same  results  by  varying  the  inductance 
and  capacitance  of  the  circuit.  In  Fig.  26  several  typical 
radio  circuits  are  shown  and  the  wave  length  to  which  these 
circuits  are  tuned  can  be  computed  from  the  relation 

X  =  1884 VLXC  (in  meters) , 

55 


56 


ELEMENTS  OF  RADIO  TELEPHONY 


where  L  represents  the  total  effective  inductance  of  the  cir- 
cuit measured  in  microhenries  and  C  is  the  total  effective 


"b 


FIG.  26.  —  Typical  radio  circuits. 


capacity  of  the  circuit  measured  in  microfarads.  Where 
both  quantities  are  lumped  together  we  may  substitute 
directly  but  in  other  cases  the  equivalent  values  must  be 
computed. 


RECEIVING  EQUIPMENT 


57 


FIG.  27. — Low- voltage  variable  air   condenser  for  receiving  circuits 
above  and  high- voltage  mica  transmitting  condenser  below. 


58  ELEMENTS  OF  RADIO  TELEPHONY 

Condensers  connected  in  parallel  are  added  directly,  while 
condensers  connected  in  series  are  equivalent  to  a  single 
condenser  whose  value  is  found  by  taking  the  reciprocal  of 
the  sum  of  the  reciprocals  of  each  of  the  individual  con- 
densers. 

Inductances  are  handled  just  the  other  way  around,  when 
in  series  they  add  directly  and  when  in  parallel  the  resultant 
is  computed  in  the  same  way  that  we  handle  series  conden- 
sers. If  one  coil  has  an  appreciable  magnetic  effect  on  the 
other  coil  there  is  another  term  called  mutual  inductance 
which  must  be  taken  into  consideration.  When  the  coils 
are  connected  in  series  the  equivalent  inductance  is  repre- 
sented by 


if  the  coils  are  wound  so  that  the  current  passes  in  the  same 
direction  in  the  two  coils.  Where  the  coils  are  connected 
so  that  the  direction  of  current  flow  is  opposite  in  the  two 
coils  the  2M  is  subtracted  instead  of  being  added.  LI  and 
Z/2,  the  self-inductances  of  the  coils,  and  M  the  mutual  induc- 
tance of  the  two  coils  may  be  computed  for  simple  cases 
from  tables.  The  effect  that  mutual  inductance  has  upon 
the  parallel  connection  of  inductance  coils  is  rather  com- 
plicated and  beyond  the  scope  of  this  volume. 

Electrical  Equivalent  of  the  Antenna.  —  The  ordinary 
elevated  antenna  is  really  a  large-sized  condenser,  one  plate 
of  which  is  represented  by  the  elevated  wires,  and  the  other 
one  by  the  ground  or  counterpoise. 

The  wires  of  the  antenna  have  inductance,  hence  the 
equivalent  circuit  would  be  represented  as  shown  by  the 
dotted  lines  in  Fig.  28.  On  account  of  the  fact  that  the  in- 
ductance and  capacity  are  uniformly  distributed  along  the 
length  of  the  antenna  it  is  not  possible  to  represent  an 
antenna  by  single  concentrated  inductances  and  condensers. 


RECEIVING  EQUIPMENT 


59 


but  values  can  be  selected  which  will  exactly  represent  the 
electrical  equivalent  at  any  single  frequency. 

Antenna  Series  Condensers  and  Inductances. — To  reduce 
the  wave  length  a  condenser  should  be  inserted  in  series 
with  the  antenna.  This  gives  a  circuit  in  which  two  con- 
densers are  connected  in  series  and  thus  reduces  the  wave 


FIG.  28. — The  electrical  equivalent  of  the  antenna. 


length  below  the  value  which  would  have  been  obtained 
with  the  natural  capacity  of  the  antenna  alone.  By  decreas- 
ing the  condenser  the  wave  length  may  be  brought  down  to 
one-half  its  natural  value  but  not  lower. 

The  insertion  of  inductance  in  series  with  the  antenna 
increases  the  effective  inductance  of  the  system  and  thus 
increases  the  wave  length.  There  is  no  limit  to  the  wave 
length  obtainable  in  this  manner. 

Some  receiving  circuits  employ  detectors  directly   con- 


60  ELEMENTS  OF  RADIO  TELEPHONY 

nected  to  the  antenna  circuit  and  others  use  a  second  circuit 
connected  to  the  detector  and  electrically  coupled  to  the 
antenna  circuit  but  insulated  from  it.  In  either  case  every 
circuit  involved  must  be  tuned  to  the  wave  length  of  the 
incoming  signal. 

Receiving  equipment  may.  be  classified  under  three  general 
subheadings,  Tuners,  Detectors  and  Amplifiers. 

Tuners. — Under  this  heading  come  such  combinations  of 
coils  and  condensers  as  are  capable  of  tuning  the  antenna 
and  associated  circuits  to  the  proper  frequency  so  that  the 
incoming  signal  may  be  received  at  maximum  intensity 
and  undesirable  signals  weeded  out.  The  choice  of  tuner 
depends  almost  entirely  upon  the  detector  employed,  so 
that  detailed  consideration  to  various  circuits  will  be  given 
under  the  discussion  of  the  various  detectors  involved. 

Detectors. — The  simplest  type  of  detector  is  the  "  crystal  " 
or  "  mineral "  detector.  It  consists  of  a  contact  between 
some  mineral  and  another  electrical  conductor  which  is 
sometimes  in  the  form  of  a  metallic  wire  and  sometimes 
another  mineral.  These  contacts  have  the  property  of 
conducting  electricity  easily  in  one  direction  and  of  shutting 
out  more  or  less  completely  any  current  that  may  tend  to 
flow  in  the  other  direction. 

There  are  quite  a  number  of  substances  which  exhibit  this 
peculiar  one-way  conduction  and  the  following  are  usually 
used  in  connection  with  metallic  points. 

Galena PbS 

Carborundum SiC 

Silicon Si 

Iron  pyrites FeS2 

Copper  pyrites CuFeS2 

Pyrolusite Mn(>2 

Molybdenite MoS2 

Chalcocite Cu2S 


RECEIVING  EQUIPMENT  61 

A  combination  of  zinc  oxide  and  copper  pyrites  known  as 
the  "  Perikon  "  detector  is  representative  of  the  second  class 
of  detectors  and  gives  very  satisfactory  results. 

There  are  a  large  number  of  good  mountings  for  crystal 
detectors  of  varying  mechanical  construction.  A  good 
mounting  should  fulfill  the  following  requirements: 

(a)  Every  part  of  the  crystal  surface  should  be  easily  and 
quickly  accessible  to  the  contact  point. 

(&)  The  pressure  of  the  contact  point  should  be  adjust- 
able from  a  very  light  contact  to  one  involving  considerable 
pressure. 

(c)  The  crystal  or  crystals  should  be  firmly  embedded  in 
the  containing  cup  and  make  good  connection  with  the  out- 
side circuit. 

(d)  The  surface  of  the  crystal  should  be  protected  from 
any  accumulation  of  grease,  dust,  and  dirt. 

In  order  to  satisfy  the  third  condition  the  crystal  should 
be  mounted  in  an  alloy  of  low  melting  point,  as  the  use  of 
solder  or  lead  is  apt  to  impair  the  sensitivity  of  the  crystal 
on  account  of  the  heat.  This  alloy  can  be  obtained  from 
radio  dealers  or  a  substitute  made  by  adding  a  few  drops  of 
mercury  to  a  small  amount  of  molten  soft  solder.  Mounting 
in  a  glass  tube  effectually  protects  from  the  action  of  grease, 
dust,  and  dirt  and  allows  an  unobstructed  view  of  the  crystal 
surface  at  the  same  time. 

Single  Circuit  Receiver  with  Crystal  Detector. — A  diagram 
showing  the  usual  connections  is  given  in  Fig.  29.  The 
incoming  signal  sets  up  a  voltage  in  the  antenna  and  this  in 
turn  produces  a  current  which  flows  through  the  circuit 
comprising  the  antenna,  tuning  coil  L,  condenser  C,  and 
ground  connection.  The  tuning  or  inductance  coil  is  usually 
adjustable  either  by  a  provision  for  changing  the  number  of 
turns  included  in  the  circuit  or  by  the  addition  of  another 
small  coil  connected  in  series  with  the  main  coil  and  arranged 


62  ELEMENTS  OF  RADIO  TELEPHONY 

so  that  it  may  he  made  to  rotate  inside  the  larger  coil.  This 
changes  the  mutual  inductance  between  the  two  coils  and 
hence  varies  the  wave  length  by  a  variation  of  the  2M  term 
in  the  expression  for  wave  length  given  on  page  58.  This 
type  of  variable  inductance  coil  is  commonly  known  as  a 
variometer  and  is  illustrated  in  Fig.  30.  Another  illustration 


Antenna, 


Phones 


FIG.  29. — Simple  crystal  detector  receiving  set. 

of  this  type  of  variable  inductance  coil  can  be  seen  in  Fig.  39, 
which  illustrates  a  set  employing  a  single  circuit  receiver  of 
different  characteristics. 

In  operating  a  set  of  this  character  the  movable  contact 
in  the  crystal  detector  is  brought  into  contact  with  the 
stationary  crystal  and  adjusted  to  its  most  sensitive  condi- 
tion by  moving  it  slightly  while  listening  in  the  telephone 
receivers.  There  will  usually  be  enough  noise  of  one  kind 


RECEIVING  EQUIPMENT  63 

or  another  present  in  the  antenna  circuit  to  adjust  the  point 
although  in  some  sets  a  small  high-pitched  electric  buzzer 
is  provided  to  set  up  an  artificial  signal  in  the  antenna  circuit. 
When  the  ciystal  detector  is  carefully  adjusted  to  its  most 
sensitive  condition  the  set  is  ready  for  tuning.  This  is 
accomplished  by  varying  the  inductance  coil  or  condenser 
over  the  entire  available  range,  listening  meanwhile  for  the 


FIG.  30. — Variometer  type  variable  inductance  coil. 

signal.  When  the  signal  is  heard,  readjust  the  detector 
point  for  maximum  clearness.  Unless  the  set  has  been 
previously  calibrated  with  the  particular  antenna  in  use  by 
means  of  a  wavemeter  the  proper  setting  is  most  easily 
obtained  by  trial. 

Theory  of  Operation. — The  current  produced  in  the 
antenna  circuit  of  Fig.  29  is  represented  at  a  in  Fig.  31.  As 
was  explained  in  the  chapter  on  modulation,  the  outline  or 
envelope  of  this  wave  corresponds  to  the  sound  wave  trans- 


64 


ELEMENTS  OF  RADIO  TELEPHONY 


mitted,  but  the  individual  pulsations  of  the  wave  are  so 
rapid  that  it  is  impossible  for  the  telephone  diaphragm 
to  follow  them.  This  wave  passing  through  the  condenser 
C  sets  up  a  voltage  wave  across  its  terminals  exactly  like 
the  current  wave  flowing  down  the  antenna  circuit  but 
slightly  behind  it  as  regards  time.  This  voltage  tends  to 
send  a  current  similar  to  curve  a  through  the  telephones,  but 


\hfoH-aqe 


across  detector. 


A  AA     Current-  through  defected. 


Volfaqe  across  phone 
condenser 


Current  through  phones. 


FIG.  31. — Currents  and  voltages  in  a  crystal  detector  circuit. 


the  detector  stops  the  flow  of  current  in  one  direction  while 
allowing  it  to  pass  in  the  opposite  direction.  The  current 
flowing  through  the  crystal  detector  will  look  like  curve  b. 
The  condenser  across  the  telephones  acts  like  an  electrical 
storage  reservoir  and  the  telephone  windings  bridged  across 
the  condenser  act  like  a  slow  leak  in  the  reservoir.  One 
of  the  fundamental  relations  in  the  case  of  condensers  is 
that  the  more  electricity  we  put  into  the  condenser  the 
greater  will  be  the  internal  pressure  tending  to  discharge 
that  electricity.  The  action  is  somewhat  similar  to  an 


RECEIVING  EQUIPMENT  65 

automobile  tire  with  a  punctured  tube.  If  we  try  to  pump 
air  into  the  tire  the  air  will  naturally  leak  out,  and  the  rate 
at  which  air  will  leak  out  through  the  puncture  will  be 
determined  by  the  rate  at  which  we  are  pumping.  In 
order  to  make  it  similar  to  the  electrical  case  we  must  pump 
at  the  same  number  of  strokes  per  minute  and  vary  the  rater 
at  which  air  is  being  delivered  to  the  tire  by  the  length  of  the 
stroke.  Consider  that  the  height  of  each  pulse  of  current 
shown  in  Fig.  316  represents  the  length  of  one  stroke  of  the 
pump.  Then  when  the  strokes  are  long  the  pressure  inside 
the  tube  will  be  increased  before  it  has  a  chance  to  leak  away 
and  on  the  other  hand  the  pressure  will  be  low  when  the 
strokes  are  short.  Curve  c  corresponds  to  what  we  might 
expect  as  regards  pressure  inside  the  tire  and  since  the  rate 
at  which  air  leaks  through  the  puncture  depends  upon 
the  pressure  of  air  inside  the  tire  curve  d  might  be  expected 
to  represent  the  rate  at  which  air  was  leaking  through  the 
puncture.  To  come  back  to  the  electrical  case,  the  pressure 
in  the  tire  corresponds  to  the  voltage  (or  electrical  pressure) 
of  the  telephone  condenser  and  the  rate  at  which  air  leaks 
through  the  puncture  corresponds  to  the  current  through 
the  telephones.  The  reason  that  curve  d  is  shown  somewhat 
smoother  than  curve  c  is  due  to  the  smoothing-out  effect  of  > 
the  inductance  of  the  telephone  windings.  Thus  we  get  a 
current  in  the  telephone  receivers  which  varies  from  instant 
to  instant  in  accordance  with  the  height  of  the  received 
wave  and  since  the  height  of  the  received  waves  is  molded 
to  correspond  with  the  desired  sound  wave,  the  telephone 
diaphragm  will  reproduce  the  desired  sound. 

Sometimes  the  condenser  across  the  telephones  is  omitted, 
the  capacity  of  the  telephone  cords  and  windings  being 
sufficient  to  store  the  requisite  amount  of  energy. 

Most  inexpensive   crystal  detector  sets   on   the  market  i/ 
employ  this  circuit  or  a  slight  modification  of  it. 


66 


ELEMENTS  OF  RADIO  TELEPHONY 


"  Loose  Coupler  "  Circuit. — The  circuit  just  considered  is 
very  good  where  there  is  no  interference  from  other  stations 
operating  on  approximately  the  same  wave  length.  Where 
such  interference  does  exist,  the  two-circuit  tuner  or  "  loose 


Fia.  32. — Crystal  detector  set  using  loose  coupler. 


coupler  "  will  give  greater  selectivity  and  "  sharper  "  tuning 
and  thus  eliminate  a  considerable  amount  of  interference. 
This  circuit  is  illustrated  in  Fig.  32.  Here  L\  and  Z/2  are 
two  inductance  coils  arranged  so  that  the  number  of  turns 
included  in  each  one  is  variable  and  so  arranged  that  the 
position  of  the  two  coils  may  be  varied  with  respect  to  each 


RECEIVING  EQUIPMENT  67 

other.  In  some  cases  the  coils  are  coaxial  and  slide  into  one 
another,  other  forms  cause  one  coil  to  rotate  inside  the  other. 
The  combination  of  the  twc  coils  and  adjusting  mechanism 
is  usually  spoken  of  as  a  "  loose  coupler,"  the  "  Navy  "  type 
indicating  the  sliding  coil  coupler  and  the  revolving  coil 
coupler  going  by  the  name  of  variometer  coupler  or  "  vario- 
coupler."  The  theory  and  operation  of  this  circuit  is  "essen- 
tially the  same  as  that  of  the  single  circuit  tuner  previously 
described  except  that  two  circuits  must  be  tuned  instead  of 
one.  After  adjusting  the  crystal  to  its  most  sensitive  point 
as  previously  described  find  from  the  table  supplied  with  the 
set  the  proper  setting  for  Z/2  and  Cz  to  give  the  desired  wave 
length.  Adjust  the  set  to  these  values  and  with  the  coils 
fairly  close  together  adjust  the  inductance  LI  until  the  de- 
sired signals  are  heard.  Then  reduce  the  coupling  byj, 
bringing  the  coils  further  apart  and  retune  if  necessary. 
A  change  in  coupling  will  change  the  wave  length  to  which  the 
set  is  adjusted,  so  the  inductances  LI  and  Lz  and  the  con- 
denser Cz  must  be  again  slightly  adjusted  to  keep  the  signal 
at  its  maximum  intensity.  It  is  always  desirable  to  use  as 
loose  coupling  as  possible,  as  interference  from  other  stations 
is  thus  reduced  to  a  minimum. 

Tube  Detectors. — The  first  vacuum  tube  detector  was  the 
so-called  "  Fleming  Valve  "  and  consisted  only  of  hot  fila- 
ment and  cold  plate.  Its  action  was  merely  that  of  a  rec- 
tifier as  has  been  described  in  a  preceding  chapter  and  it 
took  the  place  of  the  crystal  detector.  Since  it  was  no  more 
sensitive  than  a  crystal  detector  and  required  auxiliary  bat- 
teries for  its  operation,  it  never  became  very  popular  and 
hence  will  not  be  considered  here. 

The  three-electrode  tube,  however,  is  in  almost  universal 
use  where  the  distances  to  be  covered  are  great,  and  a  sen- 
sitive detector  is  desired.  A  typical  connection  is  shown 
in  Fig.  33,  here  the  circuit  that  was  formerly  connected  to 


68 


ELEMENTS  OF  RADIO  TELEPHONY 


the  crystal  detector  and  telephones  is  now  connected  between 
the  grid  and  the  filament  of  the  detector  tube. 

The  internal  action  of  the  vacuum  tube  has  been  discussed 
in  a  preceding  chapter.  There  are  two  general  explanations 
of  the  detector  action  in  a  vacuum  tube,  one  when  a  con- 


FIG.  33. — Non-regenerative  vacuum-tube  detector  set. 


•- 


denser  is  used  in  the  grid  circuit,  the  other  without  the  grid 
condenser.  Referring  to  Fig.  33  let  Ec  be  the  voltage  induced 
across  ths  terminals  of  the  condenser  €2  due  to  the  incoming 
signal. 

Represented  graphically  it  might  look  something  like 
the  wave  marked  Ec  in  Fig.  34.  If  we  consider  the  positive 
parts  of  the  waves  as  being  above  the  line,  and  those  below 


RECEIVING  EQUIPMENT 


69 


negative,  then  when  the  grid  voltage  is  positive  a  current 
will  tend  to  flow  in  the  grid  circuit  while  no  current  will  pass 
when  the  grid  is  negative.  These  pulses  of  current  passing 
into  the  grid  condenser  on  their  passage  through  the  grid 
circuit  will  charge  it  up  so  that  it  will  produce  a  negative 


Condenser 


Grid  current 


-Negative  grid 'voff-aye. 


le/ephone  current. 


FIG.  34. — Voltages  and  currents  in  vacuum-tube  detector  circuit. 

potential  on  the  grid.  In  Fig.  34  the  second  line  represents 
the  current  flowing  through  the  grid  circuit,  and  the  third 
line  the  voltage  to  which  the  grid  condenser  is  charged  from 
instant  to  instant.  The  high  resistance  across  the  grid  con- 
denser prevents  the  condenser  from  becoming  too  highly 
charged  and  allows  the  charge  to  leak  off  continuously. 
Referring  back  to  Chapter  III  we  have  seen  that  making 


70  ELEMENTS  OF  RADIO  TELEPHONY 

the  grid  more  negative  produces  a  decrease  in  the  plate  cur- 
rent, thus  the  plate  current  will  vary  as  shown  in  the  fourth 
line  of  Fig.  34.  Since  the  plate  current  passes  directly 
through  the  receivers,  the  magnetic  pull  on  the  receiver 
diaphragms  will  vary  in  accordance  with  the  current  flowing 
through  them,  and  reproduce  the  desired  sound. 

It  is  not  always  necessary  to  use  a  "  grid  leak,"  as  the  high 
resistance  placed  across  the  grid  is  usually .  called.  The- 
oretically, if  no  grid  leak  were  employed  the  grid  con- 
denser would  continue  to  charge  up  more  and  more  until 
the  grid  became  sufficiently  negative  to  reduce  the  plate 
current  to  zero  thus  "  blocking  "  the  tube  and  making  it 
inoperative,  but  practically  this  seldom  happens  in  this 
circuit,  as  even  when  high-vacuum  tubes  are  used  there  is 
usually  enough  leakage  through  the  insulation  to  prevent 
blocking,  and  where  "  soft  "  or  low-vacuum  tubes  are  used 
the  grid  can  pass  quite  a  little  current  in  the  reverse  direction, 
so  that  the  grid  condenser  never  becomes  very  highly 
charged  and  hence  makes  the  use  of  a  grid  leak  unnecessary. 

Detector  without  Grid  Condenser. — The  grid  condenser 
and  grid  leak  resistance  in  Fig.  33  could  have  been  removed 
and  a  direct  connection  made  between  the  coil  and  the  grid 
without  entirely  destroying  the  detector  action  of  the  tube. 
Although  a  tube  with  grid  condenser  usually  gives  better- 
signals  than  one  without,  yet  results  in  the  second  case  are 
in  general  almost  as  good  as  when  the  grid  condenser  is  used. 
Referring  to  Fig.  35,  the  curve  showing  the  relation  between 
grid  voltage  variations  and  plate  current,  it  will  be  seen 
that  for  a  given  positive  potential  applied  to  the  grid  the 
amount  of  the  increase  in  the  current  is  much  greater  than 
the  value  of  the  decrease  in  current,  which  the  same  value  of 
negative  grid  voltage  would  produce. 

The  signal  voltage  applied  to  the  grid  circuit  is  reproduced 
in  the  plate  circuit,  but  its  reproduction  is  distorted  due  to 


RECEIVING  EQUIPMENT 


71 


the  fact  that  when  the  grid  voltage  varies  between  equal 
positive  and  negative  values,  the  plate  current  will  show  a 
much  larger  increase  of  current  when  the  grid  is  positive 
than  it  will  correspondingly  show  decrease  of  current  when 
the  grid  is  negative.  These  fluctuations  in  the  value  of  the 
plate  current  cannot  pass  through  the  telephones  themselves 
on  account  of  the  choking  effect  of  the  large  number  of  turns 


i^roge_ 


k Currentjn  phones 


-negative  O          posfrive  — 

Vott-oge 
due  to 
signal 

FIG.  35. — Detector  action  in  vacuum-tube  detector  without  grid 
condenser. 


of  wire  of  which  their  windings  are  composed,  and  hence  they 
are  "  by-passed  "  through  the  capacity  action  of  the  tele- 
phone cords  or  through  the  condenser  across  the  telephone 
if  there  is  one.  The  current  which  flows  through  the  tele- 
phone windings  is  the  average  value  of  the  plate  current, 
and  thus  the  telephone  diaphragm  is  made  to  vibrate  so  as 
to  reproduce  the  desired  sound. 


72 


ELEMENTS  OF  RADIO  TELEPHONY 


These  various  currents  and  voltages  are  represented  in 
Fig.  35,  (a)  being  the  voltage  impressed  on  the  grid  circuit 
by  the  incoming  signal,  (6)  the  resultant  plate  current,  and 
(c)  the  current  that  will  pass  through  the  telephone  receivers. 

When  discussing  the  tube  with  grid  condenser  no  mention 
was  made  of  the  effect  of  the  distortion  of  the  signal,  and  the 
consequent  detector  action  which  it  induces,  but  to  be  exact 


Grid  Voyage 
~-neycrt-ive    O  positive* 

FIG.  36. — Grid  voltage-plate  current  curve  for  a  "soft"  detector  tube. 


both  these  functions  are  present  when  using  a  grid  con- 
denser. 

Soft  Tubes. — Tubes  that  have  an  appreciable  amount  of 
gas  left  in  them  seem  to  make  better  detectors  than  those 
which  are  thoroughly  exhausted,  and  this  increased  sensibility 
can  best  be  explained  by  considering  the  relation  between  plate 
current  and  grid  voltage  in  each  case.  Fig.  35  shows  the 
relation  between  these  quantities  for  a  "  hard  "  tube  or  one 


RECEIVING  EQUIPMENT 


73 


in  which  the  vacuum  is  very  good  and  practically  no  gas 
present.  Fig.  36  shows  the  same  relation  f or  a  "  soft  "  tube 
in  which  the  vacuum  is  not  quite  so  high  and  a  small  amount 
of  gas  remains.  The  sudden  rise  in  plate  current  when  the 
grid  is  made  positive  is  due  to  "  ionization  "  of  the  gas  par- 
ticles in  the  tube.  The  effect  of  ionization  is  to  produce  new 
electrons  from  the  gas  molecules  and  thus  greatly  increase 


FIG.  37. — One  type  of  "soft"  or  gas  content  detector  tube. 


the  plate  current  whenever  the  grid  voltage  (or  plate  voltage) 
is  brought  above  a  critical  value.  This  phenomenon  is 
called  "  ionization  by  collision  "  and  may  be  explained  in 
the  following  manner.  In  a  tube  from  which  all  gas  has 
been  exhausted  and  whose  vacuum  is  very  high,  the  elec- 
trons are  unimpeded  in  their  passage  from  the  filament  to 
the  plate  or  grid.  In  tubes  in  which  traces  of  gas  remain  this 
is  not  the  case  for  according  to  the  modern  theory  of  gases 
numerous  molecules  of  gas  are  constantly  darting  back  and 


74  ELEMENTS  OF  RADIO  TELEPHONY 

forth  inside  the  tube.  A  gas  molecule  ordinarily  comprises  a 
minute  portion  of  gas,  an  electron  or  negative  charge  and  an 
equal  and  opposite  positive  charge.  These  two  charges  are 
of  equal  value  and  opposite  polarities  so  that  their  net  effect 
is  to  make  the  molecule  electrically  neutral.  As  the  elec- 
trons move  from  filament  to  plate  some  of  them  naturally 
collide  with  some  of  the  gas  molecules.  The  force  of  the 
impact  depends  upon  the  speed  at  which  the  electron  hap- 
pened to  be  traveling.  The  velocity  of  the  electron  is 
dependent  upon  the  attraction  of  the  plate  and  grid  and  this 
in  turn  depends  upon  the  voltage  applied  to  the  plate  and 
grid.  If  the  electron  is  moving  rapidly  enough  it  will  break 
up  the  neutral  gas  molecule  and  set  free  the  negative  elec- 
tron which  was  formerly  attached  to  it.  The  positive  charge 
remains  attached  to  the  gas  particle  and  is  attracted  by  any 
negatively  charged  body  in  the  vicinity.  The  requisite 
velocity  for  the  electron  to  break  up  the  gas  molecule  is  very 
well  defined  and  when  the  plate  and  grid  potentials  are 
raised  to  the  proper  point  there  is  a  sudden  increase  in  plate 
current  of  very  considerable  value  which  produces  the  sud- 
den rise  in  current  shown  in  Fig.  36.  If  this  action  is  very 
intense  it  may  give  rise  to  a  visible  bluish  light  inside  the 
tube  familiarly  known  as  "  blue  glow." 

The  point  at  which  ionization  by  collision  begins  may  also 
be  recognized  by  listening  in  the  telephones.  As  the  plate 
voltage  or  filament  current  is  increased  a  point  will  be 
reached  at  which  a  hissing  or  frying  sound  is  heard.  This 
indicates  the  first  stages  of  ionization  by  collision  and  the 
most  sensitive  adjustment  of  the  tube  as  a  detector  is  just 
below  this  hissing  point. 

Soft  or  gas  tubes  are  very  erratic  in  action  and  although 
more  sensitive  than  well-pumped  tubes  are  frequently  dis- 
placed by  hard  tubes  on  account  of  the  lack  of  sensitive 
adjustments  and  greater  reliability  of  the  hard  tube. 


RECEIVING  EQUIPMENT  75 

A  simple  test  of  the  degree  of  vacuum  in  any  tube  may  be 
made  by  grasping  the  tube  by  the  glass  bulb  end  and  touch- 
ing any  of  the  metallic  connections  to  the  high-tension 
terminal  of  a  spark  coil  which  is  delivering  about  a  one-half 
inch  spark.  One  end  of  the  secondary  winding  should  be 
connected  to  the  primary  winding  if  not  already  connected. 
A  very  hard  tube  will  produce  no  glow  while  a  soft  tube  will 
vary  from  a  bluish  discharge  on  the  inside  surface  of  the  glass 
to  a  glow  which  may  completely  fill  the  tube  depending  upon 
the  amount  of  gas  present. 

Regenerative  Receivers. — In  Fig.  35  it  was  seen  that  the 
plate  current  consisted  of  two  parts,  one  the  low-frequency 
current  corresponding  to  the  sound  wave  which  it  was 
desired  to  receive  and  the  other  a  high-frequency  ripple  of 
the  same  frequency  as  that  of  the  incoming  radio  wave.  The 
high-frequency  ripple  in  the  plate-current  wave  is  of  no 
particular  importance  in  the  action  of  the  straight  tube 
detector,  but  can  be  utilized  by  special  means  to  enormously^ 
increase  the  strength  of  the  signal. 

In  the  chapter  on  tube  oscillators  we  have  seen  that  under 
proper  conditions  every  variation  in  the  voltage  impressed 
on  the  grid  produces  a  consequent  variation  of  the  plate 
potential,  hence  if  the  power  on  the  grid  from  the  signal 
received  on  the  antenna  is  1  microwatt,  the  plate  circuit  will 
produce  an  alternating-current  output  of  the  same  wave 
form  but  of  several  times  that  amount,  say  10  microwatts. 
If  we  arrange  the  receiver  so  that  1  microwatt  is  impressed 
back  on  the  grid  circuit  from  the  plate  circuit,  the  imme- 
diate result  will  be  approximately  twice  the  signal  strength 
in  the  telephones,  and  20  microwatts  output  of  high-frequency 
signal  energy.  Since  by  supposition  1  microwatt  in  the 
grid  circuit  produces  10  microwatts  in  the  plate  circuit 
the  high-frequency  power  in  the  plate  circuit  will  automat- 
ically double  itself  since  2  microwatts  of  power  are  now 


76  ELEMENTS  OF  RADIO  TELEPHONY 

impressed  on  the  grid,  one  from  the  antenna  and  one  fed 
back  from  the  plate  circuit.  This  doubling  of  the  power 
in  the  plate  circuit  automatically  doubles  the  amount  of 
power  "  fed  back  "  to  the  grid  from  the  plate  circuit  and  this 
in  turn  increases  the  plate-circuit  power.  This  action  is 
cumulative  and  can  thus  enonnousty  increase  the  effect 
of  the  signal  voltage  as  impressed  on  the  grid  circuit. 
There  is  one  limitation,  when  too  much  power  is  "  fed  back  " 
ifrom  the  plate  circuit  the  tube  will  oscillate  continuously 
as  we  have  seen  from  the  chapter  on  Tube  Oscillators. 
For  maximum  signal  then,  the  circuit  should  be  adjusted 
'  to  feed  back  just  as  much  power  as  is  possible  from  the  plate 
to  the  grid  circuit  without  making  the  tube  self -oscillating. 

Regenerative  Receiver  Circuits. — Regenerative  receivers 
-  may  be  of  either  single-  or  two-circuit  types  as  was  explained 
hi  connection  with  crystal  detector  circuits. 

Single-circuit  Tuner. — One  of  the  simplest  single-circuit 
schemes  is  shown  in  Fig.  38.  From  the  antenna  the  circuit 
passes  through  a  variable  condenser  C\,  down  through  the 
variable  inductance  coil  LI  to  the  ground. 

Tuning  may  be  accomplished  by  varying  either  the  con- 
denser Ci  or  the  inductance  coil  LI  or  as  is  the  case  of  a  pop- 
-*  ular  commercial  receiver  both  condenser  and  inductance  are 
varied  simultaneously.  Increasing  either  the  capacity  of 
the  condenser  or  the  inductance  of  the  coil  will  increase  the 
wave  length  to  which  the  receiver  is  tuned,  and  decreasing 
either  or  both  of  these  values  will  shorten  the  wave  length. 
The  coil  La  is  connected  in  the  plate  circuit  and  is  arranged 
so  that  its  effect  on  coil  LI  in  the  grid  circuit  may  be  varied. 
Sometimes  La  is  mounted  inside  LI  and  arranged  so  that  it 
can  rotate  inside  LI,  in  other  cases  the  coil  La  is  wound  on 
the  same  form  as  LI  and  its  effect  upon  L\  controlled  by  a 
switch  which  can  cut  in  any  number  of  turns.  The  effect 
in  either  case  is  the  same  except  that  where  the  two  coils  are 


RECEIVING  EQUIPMENT 


77 


in  fixed  relation  to  one  another,  and  adjustment  is  effected 
by  varying  the  number  of  turns,  it  is  usually  impossible  to 


FIG.  38. — Single  circuit  regenerative  receiving  set. 


obtain  as  accurate  a  control  over  the  feed  back  and  the  con- 
sequent regeneration  of  the  signal  as  is  desirable. 

In  operating  a  set  of  this  character  adjust  the  tube  filament 
current  to  the  proper  value,  then  tune  for  the  desired  signal 
by  varying  the  condenser  and  inductance  coil,  the  plate  or 


78  ELEMENTS  OF  RADIO  TELEPHONY 

"  tickler  "  coil  being  adjusted  somewhat  below  its  maximum 
value.  When  the  desired  signal  is  heard,  increase  the  plate- 
coil  adjustment  until  the  signal  is  amplified  to  its  maximum 
point.  This  point  is  just  below  the  adjustment  for  the  self- 
oscillating  condition.  A  variation  in  the  plate-coil  adjust- 
ment will  usually  change  the  tuning  of  the  antenna  circuit 
and  require  a  slight  readjustment  of  condenser  or  coil  to 
keep  the  signal  at  maximum  intensity.  Where  it  is  desired 
to  increase  the  wave-length  range  beyond  that  of  the  receiver 
alone  an  additional  combination  loading  coil  for  grid  and 
plate  circuits  is  supplied  with  some  sets. 

The  receiver  shown  in  Fig.  39  is  representative  of  the  single- 
circuit  regenerative  set.  Its  circuit  is  essentially  that  of 
Fig.  38  except  in  certain  minor  details.  The  condenser 
Ci  is  not  continuously  variable  but  is  adjustable  to  one  of 
two  values  by  interchanging  the  antenna  connection  between 
the  two  binding  posts  on  the  right-hand  side  of  the  panel. 
Coils  Li  and  Z/2  are  split  up  into  several  sections  and  are 
adjustable  by  the  use  of  a  variometer  adjustment  as  may  be 
seen  from  the  illustration.  The  large  handle  controls 
the  inductance  of  coil  Z/i  and  the  small  knob  to  the  lower  left 
simultaneously  controls  the  coupling  between  plate  and 
grid  circuits  and  the  inductance  of  coil  Z/2,  hence  the  large 
handle  would  be  used  for  tuning  and  the  small  knob  for  the 
control  of  regeneration.  The  tube  used  in  this  set  is  of 
special  design  and  requires  but  two-tenths  of  an  ampere  to 
bring  the  filament  up  to  normal  brilliancy.  The  filament  is 
of  the  coated  type  and  consists  of  a  very  fine  platinum  strip 
coated  with  the  oxides  of  certain  of  the  alkaline  metals  and 
'  will  consequently  be  ruined  if  operated  above  a  red  heat. 
The  filament  voltage  is  low  enough  so  that  an  ordinary  dry 
cell  will  supply  it  satisfactorily  and  the  current  taken  is 
so  low  that  a  single  cell  will  last  quite  a  long  time  before 
becoming  exhausted. 


RECEIVING  EQUIPMENT 


79 


filamerrf- 
rheosfaf' 


FIG.  39. — Single-circuit  regenerative  receiver. 


80 


ELEMENTS  OF  RADIO  TELEPHONY 


The  circuit  used  in  the  set  illustrated  in  Fig.  40  is  similar 
to  Fig.  38.  The  condenser  Ci  is  varied  by  the  large  knob 
on  the  extreme  left  and  the  large  knob  in  the  center  varies 


FIG.  40.— Exterior  and  interior  of  150-3000  meter  single  circuit  regenera- 
tive receiver. 


the  degree  of  regeneration.  The  coils  are  adjusted  by  the 
two-point  switch  which  short  circuits  unused  portions  to 
eliminate  dead-end  losses.  By  varying  both  condenser  and 
inductance  this  set  will  tune  from  150  to  3000  meters  without 


RECEIVING  EQUIPMENT  81 

additional  loading  coils.  The  tube  detector  and  two-stage 
audio-frequency  amplifier  are  mounted  with  the  tuner  in  the 
same  cabinet. 

The  single-circuit  regenerative  tuner  is  probably  not  as 
selective  as  the  more  complicated  two-circuit  tuner  but  will 
usually  give  better  results  in  the  hands  of  the  novice  as  it 
is  very  much  easier  to  adjust  than  the  two-circuit  tuner  unless 
the  operator  has  had  long  experience  and  will  give  a  signal 
as  loud  or  louder  than  the  two-circuit  tuner. 

Two-circuit  Tuner. — One  type  of  two-circuit  tuner  employs 
the  same  system  of  feedback  as  that  illustrated  in  the  single- 
circuit  regenerative  receiver.  Its  operation  is  quite  similar 
to  the  single  circuit  except  that  the  antenna  circuit  is  tuned 
separately  from  the  closed  or  oscillating  circuit.  The  general 
features  are  shown  in  Fig.  41.  To  tune  the  set  to  any 
desired  wave  length,  adjust  the  values  of  L2  and  €2  to  corre- 
spond to  the  wave  length  desired,  these  values  being  obtained 
from  manufacturer's  data  or  other  calibration  of  the  set, 
and  with  the  tube  in  operation  and  the  plate-coil  coupling 
adjusted  just  below  the  oscillation  point,  set  the  coupling 
between  antenna  and  tube  circuit  at  about  25  per  cent  of  its 
maximum,  and  tune  the  antenna  circuit.  The  correct 
adjustment  of  the  antenna  circuit  will  be  indicated  by  the 
loudest  signals  in  the  telephone  receivers. 

It  is  usually  advisable  to  loosen  the  coupling  between 
antenna  and  tube  circuits  as  much  as  can  be  done  without 
making  the  signal  too  weak,  as  this  adjustment  will  give 
maximum  freedom  from  interference  from  other  stations. 

After  the  coupling  has  been  loosened,  go  over  the  set  and 
readjust  all  controls  slightly  for  maximum  signal  intensity, 
as  a  change  in  coupling  will  cause  a  slight  change  in  the 
tuning  of  the  set.  The  adjustment  of  the  plate-circuit  coil 
is  very  important  as  signals  may  be  enormously  magnified 
by  its  proper  control.  In  receiving  radio  telephony  the 


82 


ELEMENTS  OF  RADIO  TELEPHONY 


adjustment  should  be  made  so  that  the  tube  has  just  stopped 
oscillating,  and  is  apparently  just  on  the  point  of  starting 
again.  When  the  tube  is  oscillating,  and  radio  telephony  is 
being  received,  a  "  beat  note  "  of  adjustable  pitch  and  usually 


7 


c, 


mm. 


Fia.  41. — Two-circuit  regenerative  receiver  with  "tickler"  coil. 


very  loud  will  be  heard  to  the  exclusion  of  the  telephone 
signals.  This  "  beat  tone  "  can  be  eliminated  by  reducing 
the  plate-coil  coupling  until  oscillations  cease,  when  the 
speech  will  be  clearly  received.  In  congested  districts  it  is 
particularly  undesirable  to  receive  radio  telephone  signals 


RECEIVING  EQUIPMENT  83 

with  the  tube  oscillating  even  though  the  beat  note  has  been 
eliminated  by  tuning  so  that  its  pitch  becomes  lower  and 
lower  until  it  finally  disappears,  as  the  oscillating  tube  sends 
out  weak  radio  waves  from  the  receiving  antenna,  which 
will  cause  considerable  interference  with  nearby  stations 
tuned  to  approximately  the  same  wave  lengths.  The  plate 
coupling  coil  LS  is  frequently  called  a  "  tickler  "  coil. 

Tuned  Plate  Receiver. — The  coupling  between  plate  and 
grid  in  the  preceding  circuits  is  principally  of  the  type 
illustrated  in  Fig.  11  and  known  as  magnetic  coupling. 
Another  type  of  receiver  which  has  attained  very  wide- 
spread popularity  in  advanced  amateur^circles  utilizes  coup- 
ling illustrated  in  Fig.  12.  The  usual  connections  are  shown 
in  Fig.  42.  The  inductances  Lg  and  Lp  are  replaced  by  two 
variable  inductance  coils  Vg  and  VP  usually  built  in  the  form 
of  variometers,  and  the  requisite  condenser  effects  are 
obtained  from  the  internal  capacities  of  the  tube  itself  and 
adjacent  conductors.  Receiving  tubes  are  usually  con- 
structed with  the  conducting  wires,  which  connect  grid 
filament  and  plate,  sealed  into  the  glass  fairly  close  together 
and  separated  by  a  layer  of  glass.  The  glass  has  high  dielec- 
tric properties  and  the  conducting  wires  separated  by  a  thin 
layer  of  glass  produce  a  considerable  capacity  effect.  The 
capacity  effects  of  the  tube  are  illustrated  in  the  diagram 
by  the  dotted  circuit  drawn  above  the  tube. 

As  would  be  supposed,  maximum  regeneration  is  pro- 
duced when  the  grid  circuit  is  tuned  to  the  same  wave  length 
as  the  plate  circuit.  If  the  capacity  between  grid  and  fila- 
ment of  the  tube  is  the  same  as  that  between  filament  and 
plate  then  the  inductance  of  the  coil  Vg  plus  the  inductance 
of  the  secondary  L2  of  the  coupler  should  be  equal  to  the 
inductance  of  the  coil  Vp.  These  capacities  are  seldom 
exactly  equal  hence  there  will  usually  be  an  unbalancing  of 
the  inductance  in  the  plate  and  grid  circuits  to  compensate. 


84 


ELEMENTS  OF  RADIO  TELEPHONY 


This  type  of  set  is  rather  complicated  to  tune,  and  different 
operators  prefer  different  procedures.  One  scheme  would 
be  to  adjust  the  two  variometers  to  approximately  the  same 
setting  or  until  the  tube  started  oscillating.  The  test  for 


FIG.  42. — Two-circuit  regenerative  receiver  with  tuned  plate  circuit. 


oscillation  is  made  by  touching  the  grid  connection  of  the 
tube  with  the  finger;  if  only  a  single  click  is  heard  in  the 
phones  either  upon  touching  the  contact  or  on  removing  the 
finger,  the  indication  is  that  the  tube  is  not  oscillating,  if 


RECEIVING  EQUIPMENT  85 

however  a  double  click  is  heard,  that  is,  a  sound  both  upon 
making  and  breaking  the  contact  with  finger,  and  if  these 
two  clicks  are  of  about  equal  intensity,  it  is  an  indication 
that  the  tube  is  oscillating.  The  test  for  oscillation  is  rather 
hard  to  describe,  the  sound  produced  is  better  described  as  a 
"  cluck  "  rather  than  a  click,  but  when  it  is  once  actually 
heard  there  will  be  no  difficulty  in  recognizing  it  afterwards. 
After  the  tube  has  been  set  into  oscillation  close  up  the 
coupling  to  the  antenna  circuit  somewhat,  and  adjust  the 
antenna  circuit  until  the  signal  is  heard.  In  adjusting  the 
antenna  circuit  to  tune  there  will  usually  be  two  points  at 
which  a  click  will  be  heard  in  the  telephones  due  to  the  fact 
that  as  the  antenna  circuit  comes  nearer  to  the  proper 
adjustment  it  absorbs  more  and  more  energy  from  the  oscil- 
lating circuit  until  oscillations  are  finally  stopped.  If  the 
antenna  adjustment  is  continued,  a  point  will  be  reached 
where  oscillations  start  again  giving  a  second  click.  The 
pcint  midway  between  the  two  settings  at  which  clicks  occur 
is  the  proper  adjustment  for  the  antenna.  The  coupling 
between  antenna  and  tube  circuit  should  now  be  reduced  to 
the  point  where  oscillations  commence  again,  and  then 
slightly  increased  until  oscillations  stop.  This  will  give  the 
best  point  for  radio  telephone  reception.  There  are  a  num- 
ber of  other  ways  of  adjusting  the  set  to  the  proper  point  for 
regeneration,  some  operators  prefer  to  reduce  the  antenna 
coupling  to  a  low  value  and  then  stop  the  oscillations  by 
mistuning  the  plate  circuit  to  the  proper  point,  this  adjust- 
ment being  very  desirable  where  considerable  interference  is 
experienced  from  stations  operating  on  wave  lengths  near 
the  one  which  it  is  desired  to  receive.  The  proper  operation 
of  a  set  of  this  character  requires  quite  a  little  experience,  and 
the  novice  cannot  expect  to  get  the  exceptionally  good 
results  that  are  possible  with  this  set  without  considerable 
experience  in  its  operation. 


86  ELEMENTS  OF  RADIO  TELEPHONY 

Amplifiers. — Where  it  is  desired  to  receive  a  signal  of 

greater  intensity  than  can  be  obtained  with  a  detector  alone 

some  type  of  amplifier  must  be  employed.     There  are  two 

f   general  types  of  amplifiers,  those  which  amplify  the  signal 

before  it  has  been  rectified  and  those  which  amplify  the  signal 

^  after  it  has  passed  through  the  detector.     The  first  class  are 

called  "  radio-frequency  "  amplifiers  and  the  second  class, 

"  audio-frequency  "    amplifiers.      Since    at    present   audio- 

,    frequency  amplifiers  are  in  the  most  extensive  use,  they 

will  be  considered  first. 

Audio-frequency  Amplifiers. — The  principle  on  which  all 
tube  amplifiers  operate  is  embodied  in  the  relation  previously 
\  discussed  whereby  any  alternating  voltage  impressed  on  the 
grid  circuit  of  a  tube  will  reproduce  itself  in  the  plate  circuit, 
enlarged  many  times.  The  usual  connection  is  shown  in 
Fig.  43,  the  primary  circuit  terminals  A  and  B  are  connected 
in  place  of  the  telephones  in  the  detector  circuit.  The  signal 
voltage  from  the  detector  is  stepped  up  in  the  transformer  T, 
and  the  secondary  voltage  is  applied  between  grid  and  fila- 
ment of  the  tube.  This  grid  voltage  controls  the  output  of 
the  plate  battery  in  accordance  with  the  impressed  signal 
and  varies  the  current  through  the  telephone  receivers. 
The  signal  produced  in  the  telephones  is  thus  increased  in 
intensity  many  times  over  the  original  signal  received  from 
the  detector.  Referring  to  Fig.  43  it  will  be  noticed  that 
part  of  the  filament  current  regulating  resistance  is  included 
in  the  circuit  between  grid  and  filament.  The  voltage  drop 
in  this  resistance  due  to  the  filament  current  flowing  through 
it  gives  the  grid  a  slight  negative  "  bias."  Amplifiers  usually 
operate  better  with  a  slight  negative  bias,  the  amount  varying 
with  the  tube,  but  usually  about  one  volt  in  value. 

If  further  amplification  is  desired  another  stage  can  be 
added  by  connecting  the  primary  terminals  of  the  transformer 
of  the  second  tube  to  the  points  marked  A'  and  B'  and  the 


RECEIVING  EQUIPMENT 


87 


FIG.  43. — Single-stage  audio  frequency  amplifier. 


6  volts        20- 100  volts 

FIG.  44. — Three-stage  audio-frequency  amplifier. 


88  ELEMENTS  OF  RADIO  TELEPHONY 

telephones  in  the  plate  circuit  of  the  second  tube.  A  three- 
stage  amplifier  is  shown  in  Fig.  44  in  which  the  same  filament 
and  plate  batteries  are  used  for  all  three  tubes.  The  use  of 
more  than  three  stages  of  audio  frequency  is  usually  not  to  be 
recommended  as  there  is  usually  a  very  pronounced  tendency 
for  the  amplifier  to  "  howl  "  or  produce  oscillations  of  audible 
frequency  continuously.  This  is  usually  due  to  some  of  the 
amplified  energy  of  the  last  stage  finding  its  way  back  to  the 
grid  circuit  of  the  first  tube.  Since  the  amplification  is  so 
high  only  a  very  slight  "  feed-back  "  coupling  is  necessary 
to  produce  the  howl  and  special  means  are  usually  necessary 
to  prevent  it.  The  most  desirable  condition  would  be  to 
have  each  stage  of  amplification  enclosed  in  a  separate  iron 
case  which  is  connected  to  ground.  This  would  prevent 
either  magnetic  or  electrostatic  coupling  between  steps,  but 
since  this  is  difficult  to  accomplish  the  different  stages  are 
usually  separated  by  grounded  metallic  screens.  It  is  usually 
desirable  also  to  ground  one  side  of  the  filament  battery  as 
well  as  the  iron  cores  of  the  amplifier  transformers. 

Audio-frequency  Amplifying  Transformers. — Audio-fre- 
quency transformers  are  usually  constructed  with  laminated 
iron  cores  of  the  closed-core  type.  The  construction  is  such 
as  to  produce  a  minimum  amount  of  electromagnetic  dis- 
turbance outside  the  core  on  account  of  the  undesirable  feed- 
back coupling  which  it  might  produce,  and  the  consequent 
howling  of  the  amplifier.  The  windings  are  composed  of 
many  thousands  of  turns  of  extremely  fine  wire  insulated 
with  an  enamel  coating,  the  total  diameter  of  the  wire  and 
insulation  being  much  less  than  that  of  a  human  hair. 
Each  layer  is  separated  from  the  next  by  a  layer  of  very  thin 
paper  and  the  whole  winding  impregnated  with  some  type 
of  insulating  compound. 

The  secondary  coil  usually  contains  from  three  to  nine 
times  as  many  turns  as  the  primary  coil,  thus  producing 


RECEIVING  EQUIPMENT  89 

a    step-up     action    on    the     voltage     impressed     on     the 
grid. 

Amplifying  Tubes. — The  general  construction  of  tubes 
used  for  amplifying  purposes  is  usually  exactly  the  same  as 
those  used  as  detectors,  with  the  exception  that  amplifying 
tubes  are  usually  exhausted  as  far  as  possible,  and  contain  a  ' 
minimum  amount  of  gas.  However,  detector  tubes  may  be 
used  for  amplification  providing  that  the  plate  voltages 
employed  are  not  sufficiently  High  to  produce  appreciable 
"  blue  glow  "  in  the  tube. 

The  detector  tubes  offered  on  the  market  will  operate 
fairly  well  as  amplifiers  if  the  plate  voltage  does  not  greatly 
exceed  the  normal  22^  volts. 

When  hard  tubes  designed  for  amplification  are  used  the 
plate  voltage  may  be  increased  considerably  with  better 
results.  The  amplifier  plate  battery  may  consist  of  as  many 
as  four  or  five  standard  22^-volt  plate  batteries  or  even  more 
in  certain  cases. 

Certain  amplifying  tubes  are  exhausted  by  the  "  chemical  " 
process,  that  is  they  are  pumped  down  to  a  fairly  low  degree  , 
of  vacuum  with  mechanical  pumps  and  then  sealed  off  at 
the  tip.  The  degree  of  vacuum  produced  by  the  mechanical 
pump  is  not  nearly  high  enough  for  tube  operation,  so  cer- 
tain chemicals  are  introduced  inside  the  tube  before  sealing 
off  which  will  take  care  of  the  remaining  gas  present.  One 
scheme  involves  painting  the  filament  support  wires  with  a 
red  phosphorus  emulsion.  Red  phosphorus  is  normally 
inert  but  when  the  tube  filament  is  lighted  the  connecting 
wires  become  heated  and  the  red  phosphorus  is  changed  to 
yellow  phosphorus  and  vaporized.  Yellow  phosphorus  and 
its  compounds  have  an  intense  affinity  for  oxygen  and  water 
vapor,  and  if  the  tube  is  in  operation  with  voltages  around 
fifty  or  above,  the  electrical  discharge  combines  the  remain- 
ing gases  with  the  phosphorus  in  a  way  not  yet  well  under- 


90  ELEMENTS  OF  RADIO  TELEPHONY 

stood.  The  metal  parts  inside  the  tube  slowly  give  up  gas 
during  the  first  few  hours  of  their  operation  and  this  gas  is 
absorbed  provided  the  plate  voltage  is  around  fifty  volts, 
but  little  or  no  action  takes  place  with  plate  voltages  below 
this  point.  Thus  if  a  tube  of  this  character  which  has  not 
been  sufficiently  hardened  is  used  with  a  22^-volt  battery, 
the  action  usually  gets  worse  and  worse  since  there  is  no  clean 
up  going  on  at  this  voltage.  The  remedy  would  obviously 
be  to  increase  the  plate  voltage  to  say  100  volts  and  operate 
it  at  slightly  above  normal  brilliancy  until  all  the  gas  has 
been  gotten  out  of  the  electrodes  and  combined  with  the 
phosphorus.  Then  the  voltage  can  be  reduced  to  22^  volts 
again  with  satisfactory  results.  If  100-volts  direct  current 
is  not  available,  the  110- volts  alternating  current  lighting 
voltage  supply  can  be  used  instead.  It  is  best  to  connect  a 
25-watt  lamp  in  series  with  one  lead  in  order  to  protect 
against  any  possible  short  circuit  that  might  occur. 

Radio-frequency  Amplification. — Amplification  of  the  sig- 
nal before  passing  it  on  to  the  detector  offers  a  number  of 
advantages.  When  radio-frequency  amplification  is  com- 
bined with  a  detector  and  audio-frequency  amplifier  it  is 
possible  to  get  an  almost  unbelievable  amplification  of  signal. 

The  general  principles  involved  are  similar  to  those  of 
audio-frequency  amplification  and  the  point  of  difference 
lies  in  the  fact  that  the  transformers  must  be  designed  for 
radio  frequencies  instead  of  audio  frequencies. 

It  is  somewhat  difficult  to  amplify  signals  of  lower  wave 
length  than  600  meters  on  account  of  the  prominent  part  the 
capacities  of  the  tube  play  in  the  circuit.  However,  with 
certain  tubes,  which  are  specifically  designed  to  give  very  low 
internal  capacities,  very  fair  amplification  may  be  obtained 
on  waves  much  shorter  than  600  meters.  The  design  of  the 
coupling  transformer  depends  almost  entirely  upon  the 
characteristics  of  the  tubes  used  and  the  wave-length  ranges 


RECEIVING  EQUIPMENT 


91 


to  be  covered.  They  contain  a  comparatively  few  turns  of 
wire  wound  on  special  iron  cores.  The  iron  used  in  these 
cores  is  rolled  very  much  thinner  than  that  used  in  audio- 
frequency transformers  and  each  sheet  is  very  carefully 
insulated  from  its  neighbor  with  a  thin  coating  of  varnish. 

Some  coupling  transformers  are  built  without  iron  cores 
and  wound  on  forms  composed  of  insulating  material,    This 


20-IOOvofrs. 


FIG.  45. — Three-stage  radio-frequency  amplifier  with  resistance  cou- 
pling connected  to  loop  antenna. 


type  requires  more  turns  than  the  iron-cored  type  but  is 
simpler  to  construct. 

Resistance  coupling  is  sometimes  employed  for  short 
wave  lengths,  and  while  not  nearly  so  efficient  as  trans- 
former coupling  has  the  advantages  of  simplicity  and  equal 
amplification  over  a  wide  band  of  wave  lengths.  A  typical 
circuit  is  shown  in  Fig.  45.  Using  a  coil  about  4  inches 


92  ELEMENTS  OF  RADIO  TELEPHONY 

in  diameter  and  containing  thirty  or  forty  turns  as  antenna 
in  the  author's  laboratory,  no  difficulty  was  experienced  in 
picking  up  signals  many  hundreds  of  miles  away  when  five 
stages  of  amplification  were  used. 

The  chief  advantage  in  radio-frequency  amplification  lies 
in  the  fact  that  several  stages  of  radio-  and  audio-frequency 
amplification  can  be  used  in  the  same  set  without  any  very 
pronounced  tendency  for  howling,  which  condition  would  be 
almost  impossible  to  avoid  if  an  equal  number  of  either  audio 
or  radio  stages  were  used. 

Telephone  Receivers. — The  telephone  receivers  used  in 
radio  differ  internally  from  those  used  in  wire  telephony 
principally  in  the  winding  on  the  electromagnets.  Radio 
receivers  are  wound  with  very  fine  wire  and  have  a  very  large 
number  of  turns  on  each  coil.  Wire  telephone  receivers 
operate  with  much  larger  currents  than  it  is  convenient 
to  obtain  in  radio,  hence  radio  receivers  have  to  have  many 
more  turns  to  compensate  for  it,  since  .1  ampere  flowing 
through  ten  turns  produces  the  same  magnetic  effect  as 
1  ampere  flowing  through  one  turn.  This  makes  the 
resistance  of  the  windings  rather  high,  usually  between 
1000  and  1500  ohms  for  each  receiver,  the  resistance  being  a 
sort  of  necessary  evil  due  to  the  small  wire  and  large  number 
of  turns  employed. 

One  type  uses  a  diaphragm  of  sheet  mica  instead  of  iron 
and  the  magnets  instead  of  acting  directly  on&hejiron  act  on  a 
light  auxiliary  armature  which  in  turn  is  connected  with  the 
center  of  the  mica  diaphragm.  Due  to  the  special  con- 
struction of  the  coil  and  magnetic  circuit,  and  the  excellent 
vibrating  qualities  of  the  mica  diaphragm,  this  type  of 
receiver  is  very  sensitive  and  will  give  a  very  loud  response  to 
strong  signals.  It  is  quite  well  suited  for  use  in  so-called 
"  loud  speaking  "  telephones. 

Unfortunately  the  most  sensitive  radio  telegraph  receivers 


RECEIVING  EQUIPMENT  93 

are  not  always  the  best  suited  for  radio  telephone  reception. 
By  properly  designing  the  diaphragms,  it  is  possible  to  make 
them  very  sensitive  to  any  certain  frequency  to  the  exclu- 
sion of  other  frequencies.  Since  the  note  which  most  modern 
spark  telegraph  stations  use  is  around  1000  cycles,  corre- 
sponding to  the  pitch  of  "  high  C,"  receivers  for  telegraph 
use  frequently  have  their  diaphragms  tuned  to  a  point  near 
1000  cycles.  This  is  very  undesirable  for  radio  telephone 
work  as  it  causes  serious  distortion  of  the  speech,  certain 
high  notes  are  very  much  exaggerated  while  others  are  badly 
suppressed,  thus  distorting  the  speech  or  music  to  a  very 
unnatural  degree.  The  more  modern  receivers  are  designed 
to  be  equally  sensitive  for  all  frequencies  within  the  audible 
range  and  have  no  one  sensitive  frequency. 

Loud  Speakers. — Where  a  number  of  persons  desire  to 
receive  radio  signals  it  is  sometimes  desirable  to  eliminate 
the  individual  telephone  receivers  and  use  a  single  loud- 
speaking  unit  in  connection  with  a  horn. 

It  is  not  possible  to  utilize  the  ordinary  type  of  iron  dia- 
phragm telephone  receivers  for  producing  very  loud  sounds 
on  account  of  the  limitations  imposed  on  the  vibration  of  the 
diaphragm  by  the  nearness  of  the  magnetic  pole  pieces. 
When  the  current  exceeds  a  certain  value  the  motion  of  the 
diaphragm  is  increased  to  such  an  extent  that  it  will  hit 
against  the  magnetic  pole  pieces  and  produce  a  rattling 
sound. 

The  Baldwin  or  mica  diaphragm  type  of  receiver,  however, , 
has  considerably  more  leeway  due  to  its  mechanical  con- 
struction, and  when  used  in  connection  with  a  suitable  horn 
will  operate  fairly  well  if  not  overloaded. 

Better  results  are  obtained  when  diaphragms  of  metal 
are  substituted  for  the  mica  diaphragms  usually  furnished 
with  this  type  of  receiver,  on  account  of  the  distortion 
present  when  mica  is  used.  The  mica  diaphragms  are  very 


94  ELEMENTS  OF  RADIO  TELEPHONY 

light  and  unduly  responsive  to  the  higher  pitched  tones, 
and  although  this  is  desirable  in  receivers  used  for  telegraphy 
it  is  undesirable  for  telephony.  One  type  of  loud  speaker 
employing  the  Baldwin  receiver  uses  a  thin  aluminum  dia- 
phragm in  which  there  is  a  series  of  concentric  circular 
depressions,  thus  greatly  reducing  the  distortion  due  to  the 
mica  diaphragms. 

Electrodynamic  Receivers. — Where  extremely  loud  sig- 
nals are  desired  such  as  would  be  required  to  fill  large  audi- 
toriums, the  electrodynamic  type  of  loud  speaker  can  be 
used. 

In  Fig.  46  the  essential  parts  of  one  type  are  illustrated. 
Instead  of  employing  permanent  magnets  electro-magnets 
are  used  to  produce  the  magnetic  field.  In  this  field  is  a 
coil  of  wire  wound  on  a  light  cylindrical  form,  and  the  sound 
currents  are  led  through  this  coil.  The  coil  and  field  are  so 
related  that  a  mechanical  force  is  exerted  on  the  coil  when 
it  is  carrying  current,  and  this  force  is  in  turn  transmitted 
to  the  diaphragm  to  which  the  coil  is  rigidly  attached.  A 
reversal  of  the  current  reverses  the  direction  of  the  pull  on  the 
diaphragm,  thus  reproducing  the  variations  of  current  in 
the  pull  exerted  on  the  diaphragm.  Referring  to  the  figure, 
the  coil  connected  to  the  6-volt  battery  actually  contains 
many  hundreds  of  turns  instead  of  the  few  showrn  and  pro- 
duces a  magnetic  field  which  passes  through  the  core  around 
which  the  coil  is  wound,  to  the  upper  end  where  it  branches 
in  all  directions  through  the  air  gap  in  which  the  moving 
coil  is  situated,  completing  the  circuit  through  the  outside 
casing  of  the  magnet.  Current  from  the  receiving  circuit 
is  led  into  the  moving  coil  through  a  pair  of  flexible  con- 
ductors not  shown  on  the  sketch.  It  can  be  seen  that  the 
moving  coil  has  almost  unlimited  freedom  of  movement, 
hence  if  the  sound  currents  are  large  the  volume  of  sound 
produced  may  be  enormous.  The  makers  claim  that  under 


RECEIVING  EQUIPMENT 


95 


favorable  conditions  sound  has  been  transmitted  a  distance 
of  several  miles  by  its  use. 

The  moving  coil  is  composed  of  comparatively  few  turns  of 
rather  coarse  wire,  hence  if  it  is  desired  to  use  it  in  place  of  a 
high-resistance  radio  telephone  receiver  a  transformer  must 
be  used  to  increase  the  current  and  lower  the  voltage  of  the 


fo  horn 


diaphragm 


Gvoti- battery 


FIG.  46. — Electrodynamic  type  of  loud  speaker. 


signal.  The  coil  on  the  transformer  with  the  larger  number 
of  turns  is  connected  in  place  of  the  radio  receiver,  and  the 
one  with  the  smaller  number  is  connected  to  the  moving  coil. 
Where  efficient  operation  is  desired  the  last  stage  of  the 
amplifier  should  contain  a  "  power "  tube  operated  at 
its  normal  plate  voltage.  The  electrodynamic  receiver  is 
capable  of  handling  an  output  of  several  watts  efficiently, 


96 


ELEMENTS  OF  RADIO  TELEPHONY 


and  the  signal  must  be  amplified  accordingly  if  maximum 
response  is  desired. 

Push-pull  Amplifier  Connection. — In  order  to  avoid  the 
usual  serious  distortion  of  speech  and  music  when  high 
amplification  is  employed  the  push-pull  circuit  has  been 
devised.  While  some  of  the  distortion  may  usually  be 
traced  to  improperly  operating  loud  speaking  devices  a  con- 


inpuf- 
circuit 


to  loud 
speaker: 


FIG.  47. — Push-pull  amplifier  circuit. 


siderable  proportion  is  due  to  the  distorting  effects  of  the 
tubes  themselves.  This  connection  which  is  illustrated  in 
Fig.  47  requires  special  transformers  with  double  windings 
and  eliminates  most  of  the  tube  distortion  by  increasing 
the  plate  current  in  one  tube  while  it  is  decreasing  the  plate 
current  in  the  other,  and  when  the  effects  of  the  two  currents 
are  taken  together  they  give  a  response  exactly  proportional 
to  the  grid  voltage.  Where  the  signals  are  weak  the  ordinaiy 


RECEIVING  EQUIPMENT  97 

single-tube  connection  gives  satisfactory  results  but  after 
they  have  been  amplified  up  through  several  stages  some 
type  of  distortionless  connection  must  be  used  in  the  last 
stages  if  clear  signals  are  to  be  obtained  from  the  loud 
speaker. 


CHAPTER  VII 
TRANSMISSION 

Antenna  and  Ground  Systems. — The  function  of  the 
transmitting  antenna  is  to  change  the  electric  power  pro- 
duced by  the  radio  transmitter  into  electromagnetic  waves 
and  to  start  these  waves  on  their  way  to  the  receiving  station. 
At  the  receiving  station  the  function  of  the  antenna  is  to 
change  the  electromagnetic  energy  which  it  absorbs  from  the 
passing  waves  into  electrical  energy  of  such  form  as  will 
operate  the  receiving  apparatus.  The  transmitting  antenna 
is  energized  by  the  radio  transmitter  and  the  energizing 
currents  which  flow  into  the  antenna  set  up  around  it  dis- 
turbances in  the  surrounding  "  ether."  Ether  is  the  name 
given  to  the  all-pervading  "  something  "  which  exists  every- 
wherej  and  which  serves  as  the  medium  through  which 
electromagnetic  waves  travel,  whether  they  be  light,  heat,  or 
radio.  Ether  bears  the  same  relation  to  electromagnetic 
waves  that  the  surface  of  the  water  bears  to  water  waves  or 
that  air  bears  to  sound  waves. 

Waves  set  up  by  mechanical  means  are  quite  similar  in 
many  respects  to  those  set  up  electrically,  so  a  brief  dis- 
cussion of  a  mechanical  case  may  be  an  aid  to  the  under- 
standing of  the  electrical  one.  Suppose  we  set  up  a  wave  in 
water  by  poking  a  stick  up  and  down  in  the  water.  The 
first  thing  that  happens  is  that  some  of  the  water  is  pushed 
aside  or  displaced  when  the  stick  is  placed  in  the  wider. 
When  the  stick  is  withdrawn  most  of  the  water  that  was  dis- 
98 


TRANSMISSION  99 

placed  flows  back  into  its  original  position,  but  in  addition 
to  this  return  of  the  displaced  water,  a  displacement  wave 
is  sent  out  in  all  directions.  The  case  with  the  radio  antenna 
is  quite  similar,  the  current  flowing  into  the  antenna  pro- 
duces an  electromagnetic  and  electrostatic  disturbance  in 
the  surrounding  ether  which  has  a  tendency  to  restore  itself 
to  the  normal  condition  just  as  soon  as  the  cause  is  removed. 
However,  when  the  current  in  the  antenna  does  finally  reach 
zero  the  disturbance  in  the  ether  does  not  entirely  collapse 
on  the  antenna  without  setting  up  a  "  disturbance  "  wave 
in  all  directions  from  the  antenna.  When  the  current  in  the 
antenna  reverses,  due  to  the  action  of  the  radio  transmitter, 
a  similar  disturbance  is  set  up  around  the  antenna  except 
that  this  disturbance  has  the  opposite  polarity  to  that  orig- 
inally produced,  and  a  portion  of  it  is  also  radiated  away 
just  as  in  the  first  case.  This  rapid  succession  of  positive 
and  negative  disturbances  are  thus  radiated  out  in  all  direc- 
tions from  the  transmitting  antenna  and  finally  impinge 
upon  the  receiving  antenna  system.  Radio  waves  have  the 
power  of  setting  up  the  same  kind  of  disturbance  in  the 
receiving  antenna  as  that  required  to  produce  them  in  the 
transmitting  antenna.  Similarly,  waves  produced  by  moving 
a  stick  up  and  down  in  water  will  tend  to  move  other  sticks 
on  which  they  have  a  chance  to  act  in  the  same  manner  that 
the  first  stick  was  moved. 

Ether  waves  travel  at  the  speed  of  light  (light  being  an 
ether  wave),  186,000  miles  or  300,000,000  meters  per  second. 
Suppose  it  takes  one-millionth  of  a  second  for  the  current  in 
the  transmitting  antenna  to  change  from  its  maximum  posi- 
tive value  through  zero  to  its  maximum  negative  value,  then 
the  positive  wave  will  have  had  one-millionth  of  a  second 
start  on  the  negative  wave  and  will  have  traveled  one- 
millionth  of  300,000,000  meters  before  the  negative  starts. 
As  they  are  both  moving  at  the  same  speed  there  will  be  a 


100  ELEMENTS  OF  RADIO  TELEPHONY 

constant  distance  of  300  meters  between  the  peak  of  the 
positive  wave  and  the  peak  of  the  negative  wave.  This 
would  correspond  to  the  distance  between  the  crest  and 
the  trough  in  the  case  of  a  water  wave  and  the  distance 
between  crests  would  naturally  be  twice  that  amount.  Sim- 
ilarly we  consider  the  wave  length  in  the  case  of  the  electric 
wave  as  being  the  distance  between  successive  positive  peaks 
and  this  distance  would  be  600  meters  under  our  assumption. 
If  it  requires  one-millionth  of  a  second  to  pass  from  positive 
through  zero  to  the  negative  maximum,  it  will  require  twice 
this  time  to  complete  a  cycle,  or  two-millionths  of  a  second. 
If  it  takes  two-millionths  of  a  second  to  pass  through  a  cycle, 
cycles  will  occur  at  the  rate  of  500,000  per  second,  in  other 
words  the  frequency  corresponding  to  a  600-meter  wave  is 
500,000  cycles  per  second.  The  relation  between  wave 
length  and  frequency  is  therefore 

wave  length  =300,000,000/frequency.     (in  meters) 

Elevated  Antenna. — For  general  reception  and  transmis- 
sion work  it  is  usually  advisable  to  erect  an  elevated  antenna 
consisting  of  one  or  more  wires.  It  is  essential  that  the 
antenna  be  carefully  insulated  from  the  ground  and  all  other 
conductors.  It  is  not  necessary  to  use  insulated  wire 
although  there  is  no  objection  to  its  use,  the  insulation  being 
usually  obtained  through  the  use  of  good  insulators  at  the 
ends  and  wherever  else  support  is  needed.  For  reception 
it  is  generally  sufficient  to  erect  but  a  single  wire  as  multiple 
wire  antennae  are  of  advantage  principally  in  transmission 
work,  and  the  expense  of  more  than  one  wire  for  receiving  is 
usually  not  justified.  The  most  desirable  characteristic  in 
any  antenna  is  height,  the  higher  the  antenna  the  greater 
the  amount  of  signal  energy  that  will  be  absorbed  from  the 
passing  wave  and  the  louder  the  signal.  Additional  length 


TRANSMISSION  101 

will  partially  compensate  for  lack  of  height  but  there  are 
limits  in  the  total  length  of  an  antenna  above  which  it  is  not 
desirable  to  go.  The  natural  wave  length  of  any  antenna 
(or  wave  length  at  which  it  will  oscillate  most  easily  without 
coil  or  condenser  in  series  with  it)  is  roughly  speaking  from  > 
four  to  five  times  the  actual  length  of  the  antenna.  The 
figure  four  applies  most  nearly  to  vertical  antennae  and  five 
to  long  low  horizontal  antennae.  By  the  use  of  a  series] 
condenser  it  is  possible  to  tune  down  to  a  value  only  slightly' 
above  one-half  the  natural  wave  length  with  one-half  the' 
natural  wave  length  as  the  lower  limit.  To  increase  the 
wave  length  we  add  inductance  coils  in  series  and  there  is  no 
limit  to  which  an  antenna  can  be  loaded  in  this  manner. 

For  example  let  us  suppose  that  an  antenna  for  reception 
on  300  meters  is  desired.  The  upper  limit  of  length  would  be 
computed  from  one  which  had  600  meters  for  its  natural  wave 
length,  and  assuming  the  four  to  one  relation,  we  find  that 
the  maximum  total  antenna  length  must  not  exceed  150 
meters  or  490  feet.  Thus,  if  we  had  two  50-foot  poles 
available  we  would  set  them  not  more  than  440  feet  apart 
and  erect  an  antenna  440  feet  long  and  about  50  feet 
high.  Actually  such  an  antenna  would  probably  be  cut 
down  to  300  feet  or  less  on  account  of  the  additional  ease  in 
tuning.  If  a  single-circuit  tuner  is  to  be  used  it  is  best  to 
follow  the  directions  accompanying  the  apparatus  as  the 
specified  wave-length  range  will  only  be  obtained  when  the 
antenna  has  the  specified  dimensions.  A  large  antenna  will 
increase  both  maximum  and  minimum  wave  lengths  to  which 
the  set  may  be  tuned  while  an  antenna  smaller  than  that 
called  for  will  decrease  both  these  values. 

In  Fig.  48  several  different  types  of  antennae  are  shown. 
The  inverted  L  is  probably  the  most  common  type  as_itjs_*. 
easy  to  erect  and  quite  efficient.     For  very  shortwave  trans- 
mission and  reception  the  T  antenna  is  quite  desirable. 


102 


ELEMENTS  OF  RADIO  TELEPHONY 


Here  we  get  twice  the  length  in  the  horizontal  portion  with- 
out producing  a  natural  wave  length  much  greater  than  that 
which  would  have  been  obtained  if  one-half  of  the  horizontal 


Inverted  "L" 


"Fan 


FIG.  48.— Types  of  antennae. 

portion  had  been  removed.  The  fan  antenna  is  well  adapted 
for  transmission  but  apparently  offers  no  particular  advan- 
tages in  reception. 


TRANSMISSION  103 

Antenna  Construction. — The  actual  construction  of  any 
antenna  will  depend  very  largely  upon  the  supports  available. 
If  one  end  is  to  be  attached  to  a  house  and  the  other  end  to 
a  tree  or  similar  support,  the  inverted  L  should  be  selected. 
An  insulator  should  be  located  between  each  support  and 
the  antenna,  and  the  antenna  will  have  to  be  swung  clear 
of  all  nearby  objects  or  else  rigidly  attached  to  them  with 
insulators.  The  insulator  near  the  house  should  be  swung 
clear  of  the  house  on  a  rope  or  wire  by  a  sufficient  distance 
to  insure  against  the  possibility  of  the  antenna  rubbing 
against  the  house  in  windy  weather.  If  one  end  is  attached 
to  a  house  and  the  other  end  to  a  tree,  allowance  should  be 
made  when  tightening  up  the  wire  so  that  the  probable  sway- 
ing of  the  tree  in  wrindy  weather  will  not  break  the  wire. 
One  way  to  get  around  this  difficulty  is  to  use  a  weight  and 
pulley  at  the  far  end  of  the  antenna.  In  any  case  the  insu- 
lator should  be  located  far  enough  out  from  the  tree  so  that 
there  will  be  no  possibility  of  any  of  the  branches  coming 
into  contact  with  the  antenna  wire. 

Masts. — If  no  trees  are  available,  wooden  or  metal  masts 
can  be  erected  without  a  great  deal  of  difficulty  or  expense. 
The  best  metal  mast  would  be  a  steel  flagpole  set  in  a  concrete 
foundation  and  can  be  purchased  from  various  manufac- 
turers, but  a  fairly  satisfactory  substitute  can  be  constructed 
from  several  lengths  of  wrought-iron  pipe.  The  lower 
lengths  should  be  of  2-  or  2^-inch  pipe  and  gradually  taper 
in  size  upwards.  That  is,  the  first  length  would  be  1\- 
inch,  the  second,  2-inch  and  so  on.  Unless  extra  heavy 
malleable  iron  couplings  can  be  obtained  it  is  usually  best 
to  have  some  machine  shop  make  up  a  series  of  couplings 
of  heavy  construction  with  holes  bored  to  fit  the  various 
sizes  of  pipe.  These  sleeve  couplings  need  not  be  threaded 
but  should  make  a  tight  fit  for  several  inches  on  each 
length  of  pipe  and  holes  should  be  bored  for  the  insertion 


104  ELEMENTS  OF  RADIO  TELEPHONY 

of  small  pins  to  keep  the  couplings  in  place.  The  weak 
points  of  such  a  mast  are  the  couplings  and  when  it  is  erected 
a  series  of  radiating  guy  wires  should  be  attached  at  each 
joint  so  as  to  remove  as  much  of  the  strain  as  possible.  A 
series  of  guy  wires  should  also  radiate  from  the  extreme  top 
of  the  pole. 

If  a  wooden  mast  is  to  be  erected  several  two  by  fours  of 
selected  clear  lumber  may  be  used.  The  lengths  should 
be  bolted  or  clamped  together  in  such  a  way  as  to  produce 
a  minimum  amount  of  weakening  at  these  points  and  the 
mast  guyed  at  each  joint  just  as  in  the  previous  case.  The 
iron  mast  should  be  given  a  good  coat  of  asphaltum  or  other 
preservative  paint  before  erection  and  the  wooden  mast 
treated  with  several  coats  of  spar  varnish  or  other  preserva- 
tive paint. 

Wire. — Either  copper,  aluminum  or  bronze  wire  may  be 
used,  the  choice  depending  somewhat  on  local  conditions. 
Aluminum  is  quite  readily  attacked  by  salt  spray  and 
chemical  fumes  and  in  this  respect  is  not  so  good  as  copper  or 
bronze,  which  are  less  subject  to  corrosion.  Soft  copper  is 
unfortunately  mechanically  weak  and  not  good  where  very 
long  spans  are  to  be  erected.  For  long  spans  phosphor  or 
silicon  bronze  or  hard-drawn  copper  should  be  employed, 
and  preferably  in  the  form  of  a  stranded  cable.  One  stand- 
ard size  in  wide  use  in  commercial  stations  is  composed  of 
seven  strands  of  No.  22  B.  and  S.  gauge  wire.  Soft  copper 
has  lower  resistance  than  any  of  the  other  types  of  wire, 
but  in  receiving  circuits  the  additional  resistance  of  the 
stronger  wires  is  usually  of  small  importance  in  comparison 
with  other  resistances  in  the  circuit.  Phosphor  bronze  and 
silicon  bronze  are  usually  hard  drawn  by  a  process  similar 
to  that  employed  in  producing  hard-drawn  copper  and  which 
produces  a  very  considerable  increase  in  tensile  strength. 
Heating  the  wire  removes  the  effect  of  hard  drawing  and 


TRANSMISSION  105 

makes  it  much  weaker  so  care  should  be  taken  that  soldered 
joints  do  not  occur  in  parts  of  the  wire  subject  to  heavy 
tension.  If  joints  are  necessary  in  the  span  proper  they 
should  be  made  with  special  twisted  sleeves  such  as  are  used 
in  making  joints  in  hard-drawn  telephone  and  telegraph  lines. 
If  a  soldered  joint  is  considered  essential  the  two  ends  of  the 
wire  can  be  slipped  through  the  sleeves  for  6  or  7  inches 
and  the  free  ends  connected  together  and  soldered.  The 
strain  is  thus  put  on  the  sleeve  and  at  the  same  time  the  wire 
is  actually  soldered. 

Copper-clad  wire  is  well  adapted  for  small  receiving  anten- 
nae. It  has  the  advantage  of  fairly  high  tensile  strength 
and  good  electrical  conductivity  at  a  reasonable  price.  It 
is  composed  of  a  steel  core  covered  with  a  layer  of  pure 
copper  and  on  account  of  the  peculiar  skin  effect  due  to  the 
high-frequency  currents  practically  all  of  the  current  is 
carried  on  the  surface  of  the  wire  and  the  steel  core  enters 
very  little  into  the  actual  electrical  conduction.  Steel  or 
iron  wire  without  the  copper  coating  is  not  to  be  recom- 
mended on  account  of  the  extraordinarily  high  losses  which 
are  incurred  when  it  is  used  as  a  high-frequency  conductor. 
Steel  and  iron  are  in  quite  general  use  on  telephone  and 
telegraph  lines  where  direct  or  low-frequency  currents  are 
to  be  carried,  but  the  skin  effect  induced  by  high-frequency 
currents  enormously  increases  the  resistance  of  iron  or  steel 
wires. 

In  choosing  the  location  for  an  antenna,  care  should  be  taken 
that  it  is  not  screened  by  other  conducting  structures.  It 
would,  for  instance,  be  very  undesirable  to  locate  an  antenna 
in  the  "  shadow  "  of  a  steel-framed  building  or  in  a  location 
thickly  surrounded  by  trees  as  a  considerable  proportion  of 
the  energy  of  the  oncoming  wave  will  probably  be  absorbed 
by  the  interfering  structure  and  the  signals  weakened  very 
much.  However  it  is  fortunate  that  during  the  winter  when 


106  ELEMENTS  OF  RADIO  TELEPHONY 

radio  transmission  is  at  its  best  the  trees  are  free  from  sap 
and  usually  frozen  and  this  naturally  greatly  reduces  the 
energy  absorbed  from  the  passing  waves.  The  absorbing 
power  of  a  hill  is  a  rather  uncertain  quantity  depending  upon 
geological  conditions;  it  is  wise  however  to  avoid,  if  possible, 
locations  near  very  steep  slopes  as  transmission  may  be 
seriously  affected. 

Grounds. — The  type  of  ground  connection  employed  will 
depend  very  largely  upon  the  condition  of  the  surface  of  the 
earth.  If  the  earth  is  very  moist  the  best  type  of  ground 
connection  is  one  consisting  of  a  number  of  metallic  stakes  or 
rods  driven  into  the  ground  directly  under  the  antenna  and 
located  so  as  to  cover  the  space  directly  under  the  antenna. 
These  stakes  should  have  copper  wires  soldered  to  them  and 
after  they  have  been  connected  together  in  a  net  work,  a 
single  lead  should  be  brought  to  the  radio  apparatus.  Where 
the  ground  is  moderately  dry  on  the  surface  it  is  well  to  see 
that  the  ground  stakes  actually  penetrate  into  the  layer  of 
permanently  moist  soil  which  is  usually  found  a  short  dis- 
tance below  the  surface.  Where  underground  water  supply 
systems  are  available  the  ground  connection  may  be  made 
directly  on  to  the  water  pipes  without  the  necessity  of  con- 
structing any  elaborate  ground  system.  Where  the  ground 
is  extremely  dry  and  no  moist  layer  can  be  found  at  any 
reasonable  depth  below  the  surface,  it  is  preferable  to  use 
a  counterpoise.  A  counterpoise  consists  of  a  replica  of  the 
antenna  erected  directly  under  it  and  located  only  a  short 
distance  above  the  ground.  No  harm  is  done  if  the  counter- 
poise system  is  larger  and  more  extensive  than  the  antenna 
system  but  it  should  be  located  as  nearly  under  the  antenna 
as  possible  and  as  near  the  earth  as  convenient.  For  receiv- 
ing apparatus  the  counterpoise  may  be  made  of  well-insulated 
wire  and  buried  a  few  inches  under  the  surface  of  the  earth 
for  the  sake  of  mechanical  protection,  but  care  should  be 


TRANSMISSION  107 

taken  that  the  insulation  does  not  become  defective  and  the 
counterpoise  become  partially  grounded,  for  a  partially 
grounded  counterpoise  is  worse  than  none.  If  the  counter- 
poise is  located  in  such  a  position  that  it  would  interfere 
with  traffic  if  located  close  to  the  ground,  it  may  be  elevated 
sufficiently  high  to  clear  pedestrians  or  teams,  but  it  is 
better  to  keep  it  as  close  to  the  earth  as  possible.  In  any 
case  the  counterpoise  must  be  well  insulated  from  the  ground. 
For  transmission  the  counterpoise  is  much  to  be  preferred 
in  every  case  except  where  open  water  or  marshy  grounds 
are  available,  but  the  ground  is  of  less  importance  in  receiving 
equipment  where  the  antenna  circuit  is  usually  of  high 
resistance  and  the  addition  of  small  amounts  of  additional 
resistance  is  of  relatively  less  effect.  When  the  antenna  is 
not  in  use  it  should  be  grounded  so  as  to  protect  it  against 
the  effects  of  lightning  and  atmospheric  electricity.  This 
is  best  accomplished  by  means  of  a  grounding  switch  located 
on  the  outside  of  the  building  and  connecting  the  antenna 
to  ground  through  a  separate  ground  wire  running  down 
the  side  of  the  building  and  terminating  on  a  stake  driven 
into  the  ground.  The  Board  of  Underwriters,  acting  for  the 
insurance  interests,  issue  from  time  to  time  specifications 
governing  the  type  and  size  of  the  grounding  equipment 
required  to  conform  to  their  regulations  and  details  may  be 
obtained  from  the  local  insurance  agents. 

Loop  or  Coil  Antennae. — The  type  of  antenna  just  dis- 
cussed is  affected  by  both  magnetic  and  electrostatic  com- 
ponents of  the  radio  wave.  In  the  case  of  loop  or  coil  anten- 
nae the  electrostatic  component  is  neutralized  and  the 
action  is  due  to  the  electromagnetic  component  of  the  oncom- 
ing wave.  The  loop  antenna  usually  consists  of  a  few  turns 
of  wire  wound  on  a  frame  of  relatively  small  dimensions. 
For  reception  from  stations  operating  on  360  meters  15 
turns  wound  on  a  frame  3  feet  square  would  be  approximately 


108  ELEMENTS  OF  RADIO  TELEPHONY 

correct.  For  longer  waves  more  turns  or  a  larger  frame 
would  be  used.  In  the  case  of  the  previously  discussed  type 
of  antenna,  the  directional  characteristics  are  not  very 
marked  unless  the  horizontal  length  is  very  long  as  compared 
with  the  height,  and  then  reception  and  transmission  are  best 
in  the  direction  opposite  to  which  the  open  end  of  the 
antenna  is  pointing.  In  all  ordinary  short-wave  antennae 
this  directional  effect  is  quite  small,  and  usually  not  notice- 
able. In  the  case  of  the  loop  antenna  the  directional  char- 
acteristics are  very  marked  and  can  be  used  to  eliminate  and 
receive  stations  at  will  depending  upon  the  direction  in  which 
their  signals  travel.  If  a  properly  connected  loop  antenna 
is  revolved  around  a  vertical  axis,  the  signal  strength  will 
vary  according  to  the  position  of  the  loop,  it  will  be  strongest 
when  the  plane  of  the  loop  is  pointing  to  the  transmitting 
station  and  will  fade  out  entirely  when  the  loop  is  moved  a 
quarter  of  a  turn  in  either  direction.  The  setting  for  zero 
signal  is  quite  sharp  but  the  point  of  maximum  signal  is 
not  well  denned  as  the  intensity  of  the  signal  is  only  slightly 
altered  by  a  considerable  angular  change  when  passing 
through  the  maximum  point.  In  determining  the  direction 
from  which  signals  are  coming,  the  loop  is  revolved  until 
the  signal  strength  is  reduced  to  a  minimum  and  at  this 
setting  the  waves  are  approaching  the  loop  in  a  direction  at 
right  angles  to  its  plane.  Ordinarily  there  would  be  no 
indication  from  which  of  two  directions  the  signals  are 
approaching;  for  instance  a  given  reading  might  indicate 
north  or  south.  When  the  exact  location  of  the  sending 
station  is  desired,  several  independent  readings  are  taken 
simultaneously  from  stations  separated  by  some  distance. 
The  readings  are  collected  and  lines  running  in  the  directions 
indicated  by  the  individual  readings  are  drawn  on  a  map 
through  the  known  locations  of  the  observing  stations  and 
continued  until  they  meet.  The  point  of  meeting  gives  the 


TRANSMISSION  109 

location  of  the  unknown  station.  A  series  of  radio  compass 
stations  equipped  with  loop  receivers  is  maintained  around 
each  of  the  principal  ocean  ports  by  the  U.  S.  Navy  and 
ships  entering  into  their  range  can  be  informed  of  their  exact 
position  in  a  very  short  time  by  simply  callirig  the  shore 
station  and  sending  a  series  of  signals.  This  system  is 
obviously  also  useful  in  locating  illegal  radio  stations. 

The  loop  antenna  can  be  used  in  place  of  the  ordinary 
antenna  by  connecting  one  end  to  the  lead  normally  con- 
nected to  ground  and  the  other  end  of  the  loop  to  the  point 
normally  connected  to  the  antenna.  A  condenser  has  to  be 
inserted  in  one  of  the  leads  since  the  loop  has  no  condenser 
action  like  the  elevated  antenna.  On  account  of  the  com- 
paratively small  portion  of  the  wave  which  is  intercepted  by 
the  loop  as  compared  to  that  absorbed  by  the  elevated 
antenna,  the  signal  given  by  the  loop  is  very  much  weaker 
than  that  produced  by  the  elevated  antenna.  Usually 
from  two  to  three  additional  steps  of  amplification  are 
required  in  connection  with  the  average  loop  antenna  to  give 
results  comparable  with  those  produced  by  elevated  an- 
tennae. 

The  one  disadvantage  of  the  loop  is  the  weakness  of  signal 
strength  and  it  has  a  large  number  of  advantages.  Weak 
signals  can  often  be  received  through  powerful  interference 
caused  by  nearby  transmitting  stations  by  turning  the  loop 
in  such  a  direction  that  no  signal  is  received  from  the  nearby 
station  without  entirely  losing  the  signal  from  the  weak  dis- 
tant station.  A  loop  also  seems  to  pick  up  less  disturbing 
noise  and  atmospherics  in  proportion  to  the  signal  produced 
than  is  the  case  with  the  elevated  antenna  and  since  the  sound 
intensity  can  easily  be  brought  up  to  any  desired  point  with 
proper  amplifiers  this  characteristic  is  of  distinct  advantage. 
Since  high  amplification  is  required  with  all  loops,  the  best 
receiver  to  use  is  one  which  comprises  several  steps  of  radio- 


110  ELEMENTS  OF  RADIO  TELEPHONY 

frequency  amplification,  a  detector,  and  several  steps  of 
audio-frequency  amplification. 

Transmission  Phenomena. — The  transmission  of  radio 
waves  through  the  ether  is  the  one  weak  and  uncertain  link 
in  the  system.  It  is  never  possible  to  accurately  predict 
just  what  transmission  conditions  will  exist  at  any  given  time-, 
during  certain  periods  signals  may  be  very  strong,  and  during 
other  exactly  similar  periods,  veiy  weak.  In  general, 
however,  it  is  found  that  signals  are  stronger  during  the 
night  than  during  the  daytime,  that  long-wave  signals  are 
less  affected  by  day  and  night  conditions,  and  that  trans- 
mission conditions  are  much  better  in  winter  than  in  summer. 
The  periods  just  around  sunrise  and  sunset  are  frequently 
noticeable  by  sudden  erratic  changes  in  signal  strength 
particularly  in  the  case  of  long-wave  long-distance  trans- 
mission. The  difference  in  day  and  night  range  is  so  pro- 
nounced that  an  amateur  station  operating  on  200  meters 
may  have  ten  times  the  range  at  night  as  compared  with  the 
daylight  range.  On  the  other  hand  signals  from  the  high- 
powered  long-wave  transatlantic  stations  show  only  slight 
variation  between  day  and  night  signals. 

Such  ranges  as  are  obtainable  on  short  waves  during  the 
daylight  are  fairly  reliable  and  the  signals  do  not  vary 
greatly  in  intensity,  while  during  the  night  the  same  station 
may  be  extremely  strong  one  instant  and  totally  inaudible 
the  next.  This  phenomenon  of  fading,  which  applies  alike  to 
radio-telephone  and  telegraph  signals,  is  not  entirely  under- 
stood, but  is  thought  to  be  due  to  reflection  and  refraction 
phenomena  in  the  upper  layers  of  the  atmosphere.  Fig.  49 
shows  a  transmitting  station  A  and  a  receiving  station  B 
located  exactly  ten  wave  lengths  apart.  Part  of  the  signal 
received  at  B  consists  of  direct  radiation  parallel  to  the 
earth's  surface  and  part  is  reflected  downwards  by  the 
reflecting  medium  R  after  having  been  originally  radiated 


TRANSMISSION  111 

upwards.  Radio  waves  being  of  exactly  the  same  nature  as 
light  waves  are  reflected  by  conducting  layers  just  as  a  mir- 
ror reflects  light  waves.  We  do  not  know  what  composes 
the  reflecting  layer  but  believe  that  it  may  be  composed 
of  dust  or  moisture-laden  particles.  If  the  distance  between 
stations  is  exactly  ten  wave  lengths,  when  a  positive  wave  is 
being  received  at  B  another  positive  wave  will  be  just  leaving 
A.  If  the  distance  happened  to  be  9^  or  10^  wave  lengths 


reflecting  surface 


FIG.  49.— Diagram  showing  how  fading  phenomena  may  be  produced 
by  reflection  from  conducting  layers  above  the  earth. 


then  a  negative  wave  would  have  been  received  at  B  while 
a  positive  wave  was  being  radiated  from  A,  or  vice  versa. 

If  the  total  effective  distance  measured  from  A  to  R  and 
back  down  to  B  is  an  integral  number  of  wave  lengths  a 
positive  wave  will  be  received  at  B  when  a  positive  wave  is 
radiated  from  A,  and  the  waves  reaching  B  over  the  two 
routes  will  add  together  and  produce  a  stronger  signal  than 
that  due  to  either  of  the  components.  The  reflecting  layer 
is  apparently  in  constant  motion  and  if  moving  upwards 
will  gradually  increase  the  length  of  the  reflected  path  until 
its  effective  length  is  increased  by  a  half  wave  length.  Now 


112  ELEMENTS  OF  RADIO  TELEPHONY 

the  wave  reaching  B  by  the  reflected  route  will  have  changed 
its  phase  with  respect  to  the  one  reaching  B  by  the  direct 
route  and  the  two  waves  will  be  of  opposite  polarity  with 
respect  to  each  other.  If  they  happen  to  be  equal  in  strength 
the  two  will  cancel  each  other  out  and  leave  no  signal  at  all. 
When  the  layer  moves  still  higher,  it  will  eventually  increase 
the  reflected  distance  by  another  half  wave  length  and  the 
two  waves  will  again  aid  each  other  and  this  action  will  con- 
tinue alternately  increasing  and  diminishing  the  signal  inten- 
sity as  the  two  component  waves  move  in  and  out  of  step. 
It  will  take  three  times  as  long  for  the  reflecting  layer  to 
move  a  distance  corresponding  to  a  half  wave  length  for  a 
600-meter  wave  as  it  will  for  a  200-meter  wave  and  observa- 
tion indicates  that  such  is  approximately  the  case,  substan- 
tiating to  some  degree  the  original  hypothesis.  Certain 
investigators  have  assumed  that  clouds  might  be  the  basis 
of  the  reflecting  surfaces,  but  observation  seems  to  indicate 
that  fading  phenomena  on  clear  nights  are  usually  more 
pronounced  than  on  cloudy  ones. 

Signals  of  all  wave  lengths  are  stronger  during  the  colder 
than  in  the  warmer  months.  This  is  particularly  true 
in  the  case  of  short-wave  signals  which  may  vary  from  being 
totally  inaudible  in  the  summei  time  to  strong  signals  in 
winter.  The  theory  that  green  vegetation  absorbs  a  large 
proportion  of  the  energy  of  the  wave  in  passing  over  the  land 
seems  substantiated  from  the  results  obtained  from  short- 
wave transmission.  Each  green  tree  and  bush  acts  as  a 
miniature  receiving  antenna  and  abstracts  a  minute  propor- 
tion of  the  signal  energy  from  each  passing  wave.  This 
absorbing  action  is  not  present  in  the  winter  months  for 
during  this  period  the  sap  is  absent  and  the  trees  frozen, 
making  them  non-conductors. 

There  seems  to  be  an  erroneous  impression  prevalent  that 
the  receiving  range  of  a  set  is  somehow  connected  with 


TRANSMISSION  113 

the  wave-length  range;  the  longer  the  wave  to  which  it 
may  be  tuned,  the  greater  the  distance  over  which  signals 
may  be  received.  This  is  a  false  idea  as  there  is  no  direct  ,. 
relation  between  wave  length  and  distance.  It  is  true  that 
where  long  distances  are  to  be  covered,  long  wave  lengths 
are  used,  but  there  is  a  different  reason  for  this.  Long 
waves  correspond  to  low-pitched  notes  while  short  waves 
are  of  higher  frequency  and  correspond  to  high  pitches.  A 
penny  whistle  is  veiy  high-pitched  but  the  factory  steam 
whistle  is  deep-sounding  and  low-pitched,  and  the  same 
general  relations  hold  good  in  the  case  of  transmitting  sta- 
tions. For  transatlantic  transmission  waves  around  10,000  > 
meters  in  length  are  employed  while  for  short-distance 
broadcasting  360  meters  is  more  suitable.  If  it  is  desired 
to  receive  transatlantic  communication,  the  receiving  set 
will  have  to  tune  up  to  10,000  meters,  but  its  long  wave- 
length range  will  not  enable  it  to  receive  broadcasting  any 
better,  and  unless  it  can  be  tuned  down  to  low  wave  lengths, 
no  broadcasting  can  possibly  be  heard. 

"  Atmospherics." — The  most  disturbing  element  in  radio 
transmission  is  the  effect  of  the  so-called  "  static  "  or  "  atmos- 
pheric "  disturbances  so  frequently  present.  Very  little  is 
known  regarding  the  origin  of  atmospherics  but  it  is  thought 
that  they  owe  their  origin  to  electrical  discharges  through 
the  air  somewhere  above  the  surface  of  the  earth.  Their 
effect  is  to  produce  irregular  sounds  of  no  particular  pitch  in 
the  telephone  receivers  and  thus  seriously  interfere  with  the 
reception  of  weak  signals.  There  are  several  different  kinds 
of  atmospherics  which  apparently  result  from  different 
causes,  a  series  of  sharp  clicks  separated  by  a  few  seconds 
probably  indicates  lightning  discharges  accompanying  a 
thunderstorm  which  may  be  many  miles  away,  a  steady 
rumble  known  as  "  grinders  "  is  usually  noted  to  a  greater  or 
less  degree  during  the  entire  summer  period  although  its 


114  ELEMENTS  OF  RADIO  TELEPHONY 

cause  is  not  definitely  known,  while  the  approach  of  a  snow- 
storm is  frequently  heralded  by  a  peculiar  hissing  and 
squealing  disturbance. 

The  elimination  of  atmospherics  is  a  most  difficult  if 
not  impossible  matter.  Tests  have  apparently  shown  that 
a  large  proportion  of  the  atmospheric  disturbances  are 
propagated  vertically,  and  this  theory  has  been  utilized  in 
connection  with  directive  reception  with  loops  to  eliminate 
some  of  their  disturbing  effects;  however,  the  extensive 
apparatus  required  makes  it  beyond  the  possibility  of  most 
amateur  stations. 

Loose  coupling  between  antenna  and  detector  circuits  in 
receivers  where  this  is  possible  will  cut  down  atmospherics  in 
a  greater  ratio  than  it  affects  signals,  and  is  about  the  only 
scheme  that  the  amateur  can  conveniently  employ.  Loop 
antennae  give  relatively  less  atmospheric  disturbance  in 
proportion  to  signal  strength  than  elevated  antennae  and 
may  be  employed  to  good  advantage  where  effective  ampli- 
fication is  possible.  Insulated  wires  laid  directly  on  the 
surface  of  the  earth  or  buried  a  few  inches  may  be  used  in 
place  of  the  regular  antenna  for  reception.  While  signals 
are  much  weaker  than  with  the  elevated  antenna  the  atmos- 
pherics are  reduced  in  still  greater  proportion  with  a  conse- 
quent gain  in  readability. 

If  the  receiving  antenna  is  located  near  other  electric  wires 
a  certain  amount  of  disturbance  will  probably  be  experienced. 
If  near  an  alternating-current  transmission  line  a  regular  hum 
may  be  induced  in  the  receiver.  Trolley  lines  will  frequently 
produce  considerable  noise  particularly  if  the  motors  are  not 
kept  in  first-class  shape.  These  noises  are  best  avoided  by 
locating  the  antenna  as  far  away  from  the  disturbing  line 
as  possible  and  preferably  at  right  angles  to  it.  A  single- 
circuit  receiver  of  any  type  will  pick  up  very  much  more 


TRANSMISSION  115 

disturbance  of  this  character  than  will  the  corresponding 
two-circuit  receiver. 

Where  trees  are  allowed  to  grow  up  into  the  high-tension 
wires  it  frequently  happens  that  the  insulation  becomes 
rubbed  off  and  an  arc  is  formed  between  the  wire  and  the 
tree.  This  will  produce  a  roar  in  the  receiver  and  is  almost 
impossible  to  eliminate.  If  the  sparking  wire  is  near  by  it 
will  usually  drown  out  all  other  signals.  The  power  com- 
panies are  usually  quite  willing  to  clear  up  such  arcing  grounds 
and  will  generally  remedy  this  difficulty  if  the  matter  is 
called  to  their  attention. 


CHAPTER  VIII 
MISCELLANEOUS 

Batteries  and  Battery  Charging. — Although  it  is  possible 
to  use  alternating  current  to  light  the  filaments  of  trans- 
mitting tubes,  it  is  usually  not  advisable  to  try  to  employ 
it  for  receiving  tubes  on  account  of  the  great  difficulties 
experienced  in  eliminating  the  alternating-current  hum. 
This  is  particularly  difficult  in  sets  employing  high  ampli- 
fication, the  hum  induced  in  the  detector  tube,  although  per- 
haps not  notice'able  without  amplifier,  may  be  extremely 
annoying  when  amplified  through  several  stages.  Direct- 
current  supply  systems  such  as  are  employed  in  farm  light- 
ing plants  and  found  also  in  the  business  districts  of  large 
cities  may  be  used  providing  the  commutator  hum  from  the 
main  generators  can  be  eliminated.  This  may  usually  be 
accomplished  with  a  condenser-inductance  filter  system  as 
has  been  described  in  the  chapter  on  tube  oscillators.  Since 
the  voltage  used  on  the  tube  is  low  compared  with  the  line 
voltage  of  the  system,  some  type  of  regulating  resistance  or 
rheostat  must  be  provided  to  take  care  of  the  difference  in 
voltages. 

The  most  satisfactory  system  is  to  use  some  type  of  bat- 
tery, as  this  will  eliminate  all  noise.  There  are  two  general 
classes  of  battery  applicable  to  this  kind  of  service,  (a) 
primary  batteries  and  (6)  storage  batteries.  The  dry  cell  is 
the  only  type  of  primary  battery  in  common  use  to-day  and 
116 


MISCELLANEOUS  117 

is  not  to  be  recommended  for  any  type  of  continuous  service 
requiring  more  than  a  small  fraction  of  an  ampere.  It  will 
satisfactorily  supply  the  so-called  "  dry  battery  "  tube  which 
requires  about  two-tenths  of  an  ampere,  but  the  standard 
tube  requiring  about  one  ampere  will  soon  exhaust  it. 

The  production  of  electricity  in  any  battery  depends  upon 
certain  internal  chemical  reactions.  As  current  is  delivered 
by  the  cell  a  portion  of  the  active  chemical  material  inside 
the  cell  is  changed  over  to  another  chemical  form,  and  when 
all  of  the  material  is  so  changed  we  say  that  the  battery  is 
discharged.  In  recharging  a  primary  battery  we  provide 
entirely  new  chemicals,  which  means  that  in  the  case  of  the 
dry  cell  the  entire  cell  is  discarded  and  a  new  one  provided. 
In  secondary  or  storage  batteries  we  can  convert  the  second 
compound  back  to  its  original  form  by  passing  current 
through  it  backwards.  A  storage  battery  does  not  store 
electrical  energy  but  gets  its  ability  to  give  back  the  elec- 
tricity put  into  it  by  a  storage  of  chemical  energy. 

Storage  Batteries. — The  electrical  size  of  a  storage  battery- 
is  usually  measured  in  amperehours.  A  40-amperehour 
batteiy  will  theoretically  deliver  40  amperes  for  one  hour, 
5  amperes  for  eight  hours  or  1  ampere  for  forty  hours. 
Practically,  the  capacity  is  reduced  as  the  current  is  increased. 
The  same  relations  hold  good  in  charging  a  battery  except 
that  on  account  of  the  internal  losses  it  is  necessary  to  put 
in  a  larger  number  of  amperehours  than  we  can  expect  to  get 
out.  The  charging  rates  and  times  are  usually  given  by 
the  manufacturer  on  the  name  plate,  in  general  the  current 
values  should  not  be  exceeded  but  no  harm  will  be  done 
by  charging  for  a  longer  time  with  a  smaller  current. 

The  length  of  time  that  any  battery  can  be  expected  to 
supply  a  set  can  easily  be  computed.  Receiving  tubes 
require  about  1  ampere  each,  hence  a  set  employing  a 
detector  and  two  stages  of  amplification  may  be  operated 


118  ELEMENTS  OF  RADIO  TELEPHONY 

continuously  for  thirteen  hours  on  a  single  charge  of  a  40- 
amperehour  battery. 

There  are  two  general  types  of  storage  battery  in  common 
use,  the  lead-acid  type  and  the  nickel-iron-alkaline  type. 

Lead-Acid  Batteries. — The  lead-acid  type  is  more  com- 
monly used  and  has  certain  advantages  and  disadvantages. 
The  advantages  include  high  electrical  efficiency  and  but 
slight  variation  of  voltage  during  charge  and  discharge. 
Constancy  of  voltage  is  of  considerable  importance  where 
critical  soft  detector  tubes  are  used  as  it  minimizes  the  adjust- 
ment necessary  as  the  voltage  drops  due  to  discharge.  Their 
disadvantages  are  great  weight,  liability  of  sulphuric  acid 
electrolyte  coming  into  contact  with  outside  materials,  and 
mechanical  weakness  of  the  glass  or  hard  rubber  containers. 
A  lead  cell  may  be  easily  ruined  by  being  allowed  to  stand 
in  a  discharged  condition. 

The  care  of  a  lead  battery  involves  charging  as  soon  as 
possible  after  discharge  and  adding  sufficient  pure  water 
from  time  to  time  to  make  up  for  that  lost  by  evaporation 
and  boiling  away  during  charging.  The  state  of  charge  is 
best  determined  by  a  battery  hydrometer  which  measures 
the  density  of  the  acid  electrolyte.  The  density  of  the 
electrolyte  will  vary  in  direct  proportion  to  the  amount  of 
charge  in  the  battery,  the  higher  the  specific  gravity  the 
greater  the  charge.  The  numerical  values  for  different  types 
of  cell  vary  with  the  design  and  are  usually  given  by  the  man- 
ufacturer. The  battery  should  be  charged  as  soon  as  the 
specific  gravity  has  reached  the  lower  point.  In  operating  a 
battery  in  radio  work  it  is  generally  quite  simple  to  recognize 
the  point  at  which  it  should  be  charged  by  the  position  of  the 
adjusting  rheostat  on  the  detector  tube,  near  the  end  of 
discharge  the  voltage  falls  off  quite  rapidly  and  the  resist- 
ance of  the  rheostat  must  be  correspondingly  decreased. 

It  is  of  extreme  importance  that  only  the  purest  water 


MISCELLANEOUS  119 

be  used  for  refilling  batteries  as  they  may  be  completely 
ruined  by  slight  traces  of  impurity.  Distilled  water  is  best 
and  can  be  procured  from  the  druggist  or  storage-battery 
service  station,  or  very  pure  rain  water  which  has  not  been 
stored  in  metallic  containers  and  is  free  from  other  contam- 
ination may  be  used.  On  no  account  should  water  from 
city  water  mains,  springs  or  wells  be  used. 

Nickel-iron  Batteries. — The  nickel-iron  battery,  more  gen- 
erally known  as  the  Edison  battery,  is  far  superior  mechan- 
ically since  it  is  mounted  in  electrically  welded  steel  con- 
tainers.    The  electrolyte  consisting  of  a  caustic  solution  is  t 
much  less  liable  to  cause  damage  with  surrounding  objects. 
It  is  practically  impossible  to  electrically  injure  a  battery  7- 
of  this  type  in  ordinary  operation.     A  periodic  complete 
discharge  which  would  ruin  a  lead  battery  actuaUy  benefits 
an  Edison  cell.     It  can  be  charged  and  discharged  at  almost 
any  rate  without  injury.     However,  the  electrical  efficiency   » 
is  low  as  compared  with  the  corresponding  type  of  lead  bat- 
tery and  the  voltage  drops  rapidly  as  it  discharges.     The 
voltage  per  cell  averages  a  little  over  1  volt  as  compared 
to  a  value  of  about  2  volts  for  the  lead  cell. 

Plate  Batteries. — The  plate  circuits  of  receiving  tubes 
require  voltages  ranging  from  20  to  100.  These  may  be 
supplied  by  generators  but  are  more  often  supplied  by  bat- 
teries. Generators  give  an  undesirable  hum  which  has  to  be 
eliminated  by  a  series  of  filters  as  previously  described. 
Both  storage  ?nd  dry  cells  are  used  for  plate  batteries. 
Certain  commercial  types  of  high-voltage  storage  battery 
are  very  satisfactory,  but  those  of  home-made  construction 
are  usually  far  from  [satisfactory.  They  are  "  messy," 
require  constant  attention  and  charging  and  in  view  of 
the  excellent  results  obtainable  with  dry  cells  are  hardly 
worth  the  additional  bother. 

Dry  batteries  for  plate  service  should  have  what  is  known 


120  ELEMENTS  OF  RADIO  TELEPHONY 

as  a  long  "  shelf  life."  The  current  taken  from  a  plate 
battery  is  so  small  that  the  battery  usually  goes  bad  from 
natural  deterioration  rather  than  from  discharging.  It 
seems  rather  difficult  to  combine  long  shelf  life  with  the 
ability  to  deliver  rather  heavy  currents  for  short  periods 
of  time  as  in  the  case  with  batteries  intended  for  flashlight 
.  service  and  hence  good  plate  batteries  are  built  by  a  formula 
quite  different  from  that  employed  in  flashlight  batteries. 
Care  should  be  taken  in  the  selection  of  plate  batteries,  some 
are  merely  assemblies  of  flashlight  batteries  while  others  are 
scientifically  designed  for  long  life  in  radio  circuits.  It  is 
usually  economy  to  buy  the  larger  sizes  of  plate  battery 
particularly  when  several  tubes  are  operated  from  the  same 
source.  The  standard  plate  battery  unit  is  one  composed  of 
fifteen  small  cells  connected  in  series  and  delivering  22  \ 
volts  normally.  Where  higher  voltages  are  required, 
several  of  these  units  may  be  connected  in  series.  An 
exhausted  plate  battery  will  give  very  peculiar  frying  and 
hissing  noises  in  the  telephones  which  are  very  frequently 
mistaken  for  atmospheric  disturbances.  Exhausted  batteries 
are  best  detected  by  the.  volt  age  test  made  with  a  voltmeter, 
the  useful  life  usually  having  been  reached  when  the  reading 
drops  to  17  volts. 

Storage-battery  Charging  Apparatus. — Since  most  storage- 
battery  charging  stations  are  rather  high  in  their  rates  for 
storage-battery  charging,  it  is  usually  economical  wherever 
possible  to  install  charging  apparatus. 

The  type  of  charging  equipment  will  naturally  depend  upon 
the  source  of  power  available.  Where  direct  current  is 
available  the  battery  can  be  connected  in  series  with  a  suit- 
able lamp  resistance  and  charged  directly  from  the  line. 
Alternating-current  circuits  cannot  be  so  used  on  account 
of  the  periodic  reversal  of  the  current,  and  some  type  of 
auxiliary  apparatus  has  to  be  employed.  Where  a  large 


MISCELLANEOUS  121 

number  of  batteries  are  to  be  charged,  a  motor  generator 
set  is  most  economical.  The  motor  is  arranged  to  be  operated 
from  the  alternating-current  line,  and  in  turn  drives  a  low- 
voltage  direct-current  generator.  For  radio  installations 
some  type  of  rectifier  is  usually  preferable  on  account  of 
lower  cost  and  additional  simplicity.  There  are  several 
types  of  rectifier,  among  which  the  mercury  arc,  the  gas- 
filled  hot  cathode  (Tungar  and  Rectigon),  and  the  vibrating 
contact  rectifier  should  be  mentioned.  Either  of  the  first 
two  types  is  very  satisfactory  and  will  operate  equally  well. 
The  mercury  arc  must  be  started  by  hand  and  will  be  per- 
manently stopped  if  the  power  is  momentarily  interrupted, 
while  the  second  type  requires  no  hand  starting  and  will 
operate  continuously  as  long  as  the  voltage  is  available. 
The  vibrating  contact  rectifier  is  not  quite  jts  satisfactory  in 
its  present  stage  of  development  as  the  other  two  types  since 
it  requires  some  adjustment  for  best  operation. 

Wavemeters. — Instruments  used  to  determine  the  length 
of  radio  waves  are  called  wavemeters.  They  consist  essen- 
tially of  an  electric  circuit  containing  both  inductance  and 
capacity,  either  or  both  adjustable  and  whose  values  are 
accurately  known.  The  additional  apparatus  required 
depends  upon  the  particular  use  to  which  the  wavemeter  is 
put. 

Measurement  of  Transmitter  Wave  Length. — In  order 
to  measure  the  wave  length  of  a  transmitting  set  some 
indicating  device  must  be  used  in  connection  with  the 
wavemeter.  This  may  consist  of  a  sensitive  high-frequency  ' 
ammeter  connected  in  series  with  the  circuit,  a  rarefied  gas 
discharge  tube  connected  across  the  terminals  of  the  con- 
denser, or  a  crystal  detector  and  telephone  receiver  connected 
to  a  condenser  terminal,  depending  upon  the  strength  of  the 
transmitting  set.  The  wavemeter  inductance  coil  is  brought 
near  some  part  of  the  antenna  circuit  of  the  transmitter  and 


122  ELEMENTS  OF  RADIO  TELEPHONY 

the  wavemeter  adjusted  until  the  indicating  device  shows 
a  maximum.  This  point  will  be  indicated  by  a  maximum 
deflection  of  the  ammeter  needle,  the  most  intense  glow  in 
the  tube,  or  the  loudest  sound  in  the  telephones.  Some  wave- 
meters  have  the  readings  in  wave  length  engraved  directly 
upon  the  scale  of  the  variable  condenser  or  inductance  and 
by  noting  the  reading  at  which  the  maximum  effect  was 
obtained  the  wave  length  may  be  directly  determined.  In 
other  cases  it  will  be  necessary  to  refer  to  the  calibration 
chart  which  shows  the  relation  between  wave  lengths  and 
scale  readings.  The  theory  is  that  the  maximum  effect 
will  be  produced  when  the  natural  frequency  of  the  wave- 
meter  circuit  is  the  same  as  that  of  the  wave  to  be  measured, 
and  thus  the  unknown  frequency  can  be  computed  by  know- 
ing the  capacity  and  inductance  of  the  wavemeter  circuit. 

When  measuring  the  wave  length  of  continuous  wave 
transmitters  such  as  tube  telegraph  and  telephone  sets  care 
must  be  taken  that  the  coupling  between  the  wavemeter 
and  the  set  is  not  too  great  as  the  tuning  is  extremely  sharp 
and  a  change  of  one  or  two  degrees  in  the  setting  of  the 
instrument  may  be  sufficient  to  throw  the  pointer  off  the 
end  of  the  scale  and  burn  out  the  instrument  if  the  wave- 
meter  is  too  near  the  transmitting  set. 

Receiving  Circuit  Calibration. — For  measurements  with 
receiving  apparatus  the  wavemeter  is  usually  arranged  to 
emit  feeble  electric  waves  of  known  length  which  are  received 
on  the  receiver  just  as  waves  coming  in  on  the  antenna.  To 
find  the  adjustment  on  the  receiver  for  any  given  wave 
length,  set  the  wavemeter  to  send  out  signals  of  this  fre- 
quency and  then  adjust  the  receiver  until  the  loudest  sound 
is  heard  in  the  telephones.  The  wave  length  of  any  distant 
transmitting  station  can  be  measured  by  adjusting  the 
receiving  set  to  receive  the  signal  from  the  station  with 
maximum  intensity  and  then  without  disturbing  the  receiver 


MISCELLANEOUS 


123 


adjustment  vary  the  wave  sent  out  from  the  wavemeter 
until  the  loudest  sound  is  heard  in  the  telephones  when  the 
setting  on  the  wavemeter  will  naturally  give  the  wave  length 
of  the  distant  station.  In  order  to  emit  signals  a  high- 


ammefer 


buzzer 

FIG.  50. — Typical  wavemeter  connections. 


pitched  buzzer  is  used  and  the  usual  connection  is  shown  in 
Fig.  50.  The  rapid  interruption  of  the  battery  current 
each  time  the  contact  on  the  buzzer  is  broken  sets  up  pulses 
of  batteiy  current  through  the  inductance  coil  of  the  wave- 


124 


ELEMENTS  OF  RADIO  TELEPHONY 


meter  and  this  sets  the  circuit  into  oscillation  at  its  own 
natural  wave  length  just  as  plucking  a  mandolin  string 
produces  a  sound  whose  pitch  is  determined  by  the  tension 
and  size  of  the  string. 


FIG.  51. — Kolster  decremeter  and  wavemeter. 


The  Kolster  Decremeter  and  Wavemeter  illustrated 
serves  a  number  of  purposes;  measurement  of  the  wave 
length  of  a  transmitter  can  be  made  with  the  high-frequency 
Himnrter  or  the  crystal  detector  and  telephones,  while  the 


MISCELLANEOUS  125 

buzzer  supplied  by  the  small  dry  battery  attached  to  the 
side  of  the  instrument  enables  it  to  send  out  waves  of  known 
wave  length  for  receiver  calibration.  The  measurement  of 
decrement  from  which  the  instrument  gets  its  name  is  of 
interest  principally  in  the  case  of  spark  transmitters  used  for 
radio  telegraphy  and  is  made  to  determine  the  sharpness 
with  which  a  given  spark  station  may  be  tuned  in  or  out. 
The  Government  regulations  specify  certain  requirements 
in  this  regard  to  prevent  undue  interference  with  other  sta- 
tions. There  are  three  removable  coils  supplied  with  the 
instrument  and  the  condenser  is  continuously  variable 
with  values  of  wave  length  corresponding  to  different  set- 
tings of  the  condenser  engraved  directly  upon  the  condenser 
dial. 

The  Selection  of  Apparatus  for  Broadcasting  Reception. — 
Just  as  the  distance  over  which  a  sound  may  be  heard 
depends  upon  the  loudness  of  the  sound,  the  acuteness  of 
the  listener's  ear,  and  the  amount  of  other  noise  present,  the 
range  of  any  radio  telephone  broadcasting  station  is  depend- 
ent upon  similar  factors.  Up  to  a  distance  of  150  to  200 
miles  from  such  first-class  stations  as  Pittsburgh,  Newark, 
Schenectady  and  Detroit,  the  problem  is  one  involving 
almost  entirely  the  acuteness  or  sensitivity  of  the  receiving 
apparatus.  Beyond  this  distance  the  signals  are  often  seri- 
ously interfered  with  by  outside  noises  such  as  "  atmos- 
pherics "  and  noises  produced  by  local  electrical  apparatus, 
so  that  the  question  is  no  longer  one  of  sensitivity  but  of 
selectivity. 

It  is  very  difficult  to  predict  with  any  degree  of  certainty 
just  what  any  given  installation  will  do  on  account  of  the 
great  difference  in  the  receptive  powers  of  different  antennae. 
Under  average  conditions  with  an  antenna  about  40  feet 
in  height  and  around  150  feet  long  it  should  be  possible 


126  ELEMENTS  OF  RADIO  TELEPHONY 

to  obtain  satisfactory  results  with  either  a  single  or  coupled 
circuit  type  of  crystal  detector  receiver  from  any  station 
of  the  class  mentioned  above  up  to  distances  cat- about 
30  miles.  Satisfactory  results  have  been  attained  with(afer 
this  equipment  at  much  greater  distances  but  these  results^ 
somewhat  exceptional. 

Where  greater  sensitivity  is  desired  for  locations  up  to 
100  miles  some  type  of  vacuum  tube  detector  is  the  most 
desirable.  The  non-regenerative  circuit  is  very  much  less 
sensitive  than  the  regenerative  circuit  and  care  should  be 
taken  when  purchasing  apparatus  to  find  out  just  what  circuit 
is  employed  as  certain  of  the  non-regenerative  sets  look 
almost  exactly  like  the  better  regenerative  receivers. 

Every  tube  receiver  requires  two  batteries  or  their  equiva- 
lent, one  to  light  the  filament  and  the  other  to  supply  a 
direct-current  voltage  around  22  volts  for  the  plate.  If 
standard  tubes  employing  tungsten  filaments  are  used,  it  will 
probably  be  necessary  to  purchase  a  storage  battery  for  the 
filament  supply.  One  set  of  this  character  uses  a  special 
tube  with  low  filament  consumption  and  this  tube  may  be 
supplied  from  a  single  dry  battery. 

Increase  in  sensitivity  beyond  this  point  is  obtained  by  the 
addition  of  one  or  more  steps  of  amplification.  If  an  outside 
antenna  of  usual  dimensions  is  used  a  regenerative  receiver 
and  two-stage  audio-frequency  amplifier  will  usually  receive 
about  all  that  can  be  received  as  such  a  set  will  amplify  the 
disturbing  noises  up  to  a  point  where  they  interfere  with  the 
distant  signals  and  will  thus  be  capable  of  reproducing  any 
signals  that  can  be  heard  above  the  local  atmospherics  and 
other  disturbing  noises. 

Indoor  Antennae. — It  is  possible  to  erect  indoor  antennae 
and  obtain  approximately  the  same  results  as  would  have 
been  obtained  with  the  same  antenna  erected  outside  the 


MISCELLANEOUS  127 

building  provided  that  the  construction  does  not  shield  the 
antenna  from  the  incoming  waves.  Theoretically  if  the 
building  contains  no  metal  of  any  kind  the  signals  should  be 
just  as  strong  with  an  indoor  antenna  as  they  would  be  with 
an  outdoor  one  of  the  same  dimensions.  This  condition 
is  seldom  found  in  practice  as  nearly  every  structure  has 
some  metallic  circuits  such  as  electric  wires,  water  pipes,  etc., 
but  if  they  do  not  extend  above  the  antenna  their  effect  is 
not  great. 

Equipment  using  Loop  Antennae. — Loop  antennae  can  be 
substituted  for  outside  antennae  with  very  satisfactory 
results  if  sufficient  amplification  can  be  provided.  As  a 
general  rule  a  loop  of  average  dimensions  requires  about 
two  or  three  stages  of  additional  amplification  to  produce 
the  same  signal  as  would  be  obtained  from  an  average- 
sized  elevated  outdoor  antenna.  Where  extremely  high 
amplification  is  necessary  it  is  usually  necessary  to  utilize 
both  radio-  and  audio-frequency  amplification.  It  is  pos- 
sible to  obtain  completely  self-contained  receivers  consisting 
of  loop  antenna,  radio-  and  audio-frequency  amplifiers, 
detector,  and  loud  speaker  assembled  in  a  single  cabinet. 
This  makes  an  ideal  combination  as  the  set  can  be  located 
almost  anywhere  as  it  is  self-contained  and  requires  no  out- 
side antenna.  There  are  certain  advantages  of  the  loop  as 
compared  with  the  outside  antenna  which  have  been  pre- 
viously discussed  which  can  be  realized  in  this  type  of 
receiver.  The  principal  disadvantages  are  the  high  initial  ^ 
cost,  heavy  drain  on  the  storage  batteries  and  rather  complex 
circuits  involved. 

Loud  Speakers. — Loud-speaking  telephones  are  desirable 
where  several  persons  listen  simultaneously.  They  require 
however  from  one  to  two  additional  stages  of  audio-fre- 
quency amplification  with  the  last  stage  preferably  employing 


128  ELEMENTS  OF  RADIO  TELEPHONY 

the  "  push-pull  "  connection  and  with  power  tubes  sub- 
stituted for  ordinary  receiving  amplifier  tubes.  The  elec- 
trodynamic  type  of  loud  speaker  requires  a  separate  circuit 
from  the  storage  battery  to  energize  the  electromagnet  in  the 
apparatus. 


INDEX 


Airplane  generator,  35 
Alexanderson  alternator,  12 
Alternation,  6 
Alternator  transmitters,  12 
Amplification 

constant  of  tube,  21 

mechanical,  20 
Amplifiers,  86 

audio-frequency,  86 

power,  42 

push-pull,  96,  128 

radio-frequency,  90 
Amplifying  transformers,  88,  90 
Amplifying  tubes,  89 
Antenna 

coil,  107,  127 

connections,  45 

construction,  103 

effect  of  loading,  59 

elevated,  100 

equivalent  circuit  of,  58 

indoor,  126 

location,  105 

loop,  107,  127 

masts,  103 

theory,  98 

types  of,  102 

wire,  104 


Atmospherics,  113 
elimination  of,  114 

B 

Baldwin  receiver,  93 
Batteries,  116 

dry,  117,  119 

plate  circuit,  119 

primary,  116 

storage,  117 
lead-acid,  118 
nickel-iron,  119 
Battery  charging,  120,  121 
Broadcasting 

selection  of  apparatus  for,  125 


Choke  coil,  radio-frequency,  41 
Circuits 

receiver — see  under  Receiver 

transmitting  tube,  44 
Compass,  radio,  109 
Condenser 

connections,  33  . 

filter,  41 
Conductor 

definition,  14 
Coil  antenna,  107 


129 


130 


INDEX 


Counterpoise,  106 
Current 

alternating,  6 

direct,  6 

high-frequency,  7 

plate,  18 

saturation,  18 
Cycle,  6 


Decremeter,  124 
DeForest,  Dr.  Lee,  19 
Detector 

crystal,  60,  61 

"soft"  tube,  72 

vacuum  tube,  67 
with  grid  condenser,  68 
without  grid  condenser,  70 
Disturbances,  113,  114 
Dynamotor,  34 


E 

Electricity 

negative,  16 

positive,  16 
Electrons,  14 

mutual  repulsion  of,  17 

value  of,  15 
Ether,  98 

waves,  99 
Evaporation 

of  electricity,  15 

of  water,  15 


Fading,  111 
Feedback 

coupling,  types  of,  28,  29,  30,  31 

principle,  27 


Filaments 

oxide-coated,  21 

tungsten,  15,  21 
Filter 

condensers,  41 

systems,  37 
Fleming  valve,  16 

rectifier  action  of,  17 
Frequency 

chart,  8 

definition,  6 

formula,  32 

radio,  7 

tube  oscillation,  28,  32,  33 

G 

Generators 

direct-current,  33 

dynamotor,  34 

high-voltage,  35 

location  of,  44 
Goldschmidt  alternator,  12 
Grid  electrode,  19 

action  of,  24 
Ground  connection,  106 

counterpoise,  106 


Heat  waves,  8 
Howling  telephone,  27 

I 
Inductance 

formula,  58 
lonization  by  collision,  39,  74 


Joly-Arco  alternator,  12 


INDEX 


131 


Light  waves,  8 

Loop  antennae,  107 

Loud  speakers,  92,  93,  127 


Magnetic  coupling,  29,  33 
Magnetic  modulator,  13,  48 
Master  oscillator,  43 
Masts,  103 
Modulation 

antenna  control,  47 

"constant  current,"  51 

grid  circuit,  52 

plate  circuit,  50 

tube  outfit,  50 
Modulator 

magnetic,  13,  48 

systems,  47 
Molecule 

activity  of,  14 

O 

Oscillations 

frequency  of,  28,  32,  33 

production  of,  27 
Oscillator 

master,  43 

vacuum  tube,  24 


Plate  voltage,  usual  values  of,  33 


R 

Radio  compass,  109 

Receiving  circuit  calibration,  122 

Receiving  equipment,  55 


Receivers 
Baldwin,  93 
crystal  detector 
single  circuit,  61 
"loose  coupler,"  66 
electrodynamic,  94 
loud-speaking,  92,  93 
telephone,  4,  94 
tube 

non-regenerative,  67-75 
regenerative,  75-85 
Rectifiers 
cold  cathode,  39 
double-wave,  36 
electrolytic,  39 
hot  cathode,  36 
internal  action  of,  37 
single-wave,  36 
systems,  36 


"Soft"  tubes.  72 
Spark  transmitters,  10 
Steam  hammer,  24 


Telephone 

diaphragm,  2 

radio,  4 

receivers,  4,  92 

systems,  comparison  of,  1 

transmitter,  1 
"Tickler"  coil,  78,  83 
Transmission  phenomena,  98,  110 
Tubes 

amplifier,  89 

"soft,"  72 
Tuners,  60 


132 


INDEX 


W 

Wave-length 
antenna,  101 
formulae,  32,  55,  56 
measurement  of,  121,  122 
relation  of,  to  range,  112 

Wavemeters,  121 

Waves 
damped,  9 
electromagnetic,  98 


Waves — Continued 

heat,  8 

light,  8 

sustained,  9 
Wire 

antenna,  104,  105 

"litzendraht,"  48 


X-rays,  8 


I 


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